Does camping and tunneling need a valid reason ?

Your question is based on a false premise. The only reason you need to justify Tunneling, Camping, Slugging, etc. is that you decide to do it. They are legal, valid tactics within the rules. You don't need justifications. If you want to do it, or feel it is important to do it to win a given match, you should feel free. There is no Survivor's Guide for Killers. That is a joke, and only funny because so many bad sport Survivors seem to think it exists (or should). You do you. Only you know what you need to do at any given moment. If YOU want to set rules for yourself, feel free to do so. Just remember that whatever morays you apply to yourself do not apply to others, nor should you judge anyone else if they choose not to adopt your personal code. The only things anyone has to do to be a good sport in Dead by Daylight are:

1. Don't cheat, either by hack or lag spike.
2. Don't disconnect; if you start a game, finish it.
3. Be humble in victory, and gracious in defeat.

If the DEV ever decide they want something against the rules, they will tell us and/or create mechanics to appropriately charge us for the activity. Tactics are not strategies; they are tools. Sometimes Tunneling and Camping are the right call, and sometimes they will hurt you more than help you. Used correctly they are potent; used incorrectly they are no different from shooting yourself in the foot. Only time and practice will hone your instincts to know when one is right and when one is wrong.

I needs to be stated that newbie Killers tend to face newbie Survivors. New Survivors are weaker than new Killers because the strength of Survivors is in their cooperation. They don't know how to work Generators to milk hooks, how to deal with a camper via Perk or coordination. They don't know any of the slightly more complex tactics required to beat the brutal, simple approach of camping and tunneling. They will learn, but it will take them longer. That is why we get so many damn complaints here. It feels very unfair to them, because they don't know how to do anything yet. The real irony of the matter is that newbie Killers should NOT Camp or Tunnel because it hurts them in the long run. The balance of power eventually shifts, like a see-saw, to the Survivors. Once they know what they should be doing, Killers are always behind the Eight Ball. Does that make sense? Newbie Killers SHOULD be taking advantage of the easier, early games to hone hunting skills. You need to know how to Search & Destroy. Trying to learn it later when Survivors know what they are doing is painfully difficult. :)

Yep. If Survivors kept bringing it to the same degree as before, Killers would exercise more caution. :)

Most likely for a couple of reasons:

1. Perhaps they never learned how to Search & Destroy well, and thus feel constrained to doing it. This isn't unlike Survivors that never learn to Loop, so they only evade (or vice versa). A Killer that doesn't learn to do multiple things well, will lean into the only thing they think they can do with competence.
2. The Survivors make the mistake of rewarding the tactic. Survivors who rush the hook and/or don't milk hooks for Generator progress tend to make the tactics better than they would be otherwise. Survivors that make unsafe rescues, don't take protection hits, trade hooks, don't do shoulder rescues, sabotage hooks, or steal chases shrink the scope of the Survivor options. Those options are the counter play to tunneling and camping. Most of what the Killer must do is in response to what Survivors require of them.
3. And some Killers simply have a long term strategy of the 3-Generator Lock from the very start. Bear in mind a 3-Gen is a STRATEGY. Tunneling, Camping, Slugging, etc. are just tactics. However, if you are carefully and methodically going to defend a 3-Gen, you well end up camping a certain section of the map and therefore camping anyone you hook within that territory. It means, since you don't want to stray from it, that you will tunnel out of necessity to maintain what little pressure you can by keeping at least one Survivor in it and/or removing them from the game as rapidly as possible. You get the extra pressure from people who might TRY the rescues. The 3-Gen isn't an automatic win button. Good Survivors know how to avoid a 3-Generator, but they must be mindful. Killers attempting to keep that 3-Generator plan can shoot themselves in the foot because if it gets broken suddenly, they have given up a lot on a late game bet.

I agree with you to a degree, but it isn't always wrong on the first down. I tend to do short range patrols and proxy-camp on the first down simply to test how altruistic the Survivors are as I consider that valuable information for the rest of the match. Once I know how eager they are for the save as compared to staying on mission to milk that hook, I can play more effectively. Only on the first down do I have the the leeway to really feel that out. But it is really a matter of play style I think. Face camping on the first hook it just idiotic, however, as it gives up all pressure, tests nothing, and just encourages the Survivors to bang the Generators out.

Nope.

Play how you wanna play.