Why do you feel that "you're welcome" implies an obligation? It means just the opposite. If someone thanks you for giving them something, you are telling them "oh, you are welcome to take it", which means that they don't owe you anything for it.
Because my observation/experience is that in American culture, most people operate from a default assumption of one up/one down and are constantly jockeying for their position in pecking order. I get this type of thing a lot less from other cultures. But quite a lot of Americans seem unable to interact as equals and are constantly trying to force the other person into a one down position in order to avoid being in the one down position themselves. I am painfully aware of this because, frequently, if I refuse to take the one down position and try to interact as equals, they react like I am trying to insult and oppress them and force them into the one down position -- many people seem unable to conceive of some other paradigm. Saying "You are welcome" suggests "I provided you with something of value, thus I am one up on you." -- at least to most people I interact with. I do a lot of things to try to disengage from this process of jockeying for position. I dislike it intensely.