Well, let's take this analogy:
- You go to the beach, where everyone is free to play with the sand;
- You make a beautiful big sand castle which took you the entire day to make, which obviously made you very tired;
- Some lazy random dude comes and pushes you, takes your sand castle, adds just a crappy door and now says the castle is his.
How would you feel? This one was for credits, now take the next one for permissions:
- You're at home and you leave the door open;
- Some random dude enters it, eats the food which was in your fridge, and goes sleep in your bed;
- You wake him up, and then you ask: what the hell, why are you eating my food and sleeping in my bed?
- And he answers: well, you left the door open, the fridge unlocked and the bed available, so I guess I have the right to use all these for myself regardless of your opinion no?
How would you react if someone just used your stuff in your home just because you left the door open? This one was about permissions.
In a more technical view:
If I make a texture, a model, code, whatever, it's MY property, as without me it wouldn't even exist, and that property obbeys MY rules in how it should be used. So if I make a readme stating that everyone can use and stuff, then it's ok to use because I am letting you use it without asking by what I wrote.
However, if in another hand I don't include one and you know its origin or if I include one and I ask credits or/and require permissions, you just have to obey and respect it, independently of your personal opinion, as otherwise it's nothing more than theft, and it's not different from stealing stuff from the market for example.
Just also think that this stuff is made free of charge, a good share of it by people who work hard every single day of the week to have money for food, bills and to take care of his/her family, and then which may sacrifice a good share of their already really slim free time to do these things for everyone, which otherwise people would have to pay for from professionals, and the only thing they ask in return is to either ask first or to include credits.
It all goes down to values and morality, and respect for the actual work made, and the ones who don't respect it either go to jail for stealing (copyrighted stuff) or get banned (if not worse) from every possible big place they could show off their work (take Kaal for example) or other not so good consequences.
Also think that some developers may just quit doing things for a game if their work gets to be stolen.... then who will the random dude get his stuff from? The game would instantly stall and die eventually (in a very short timespan).
If the game is alive for over a decade is not thanks to the players alone, I would dare to say 75% of the credits for it would be for the hard working developers.