Why does the rating rise when you win a weaker player

I was wondering, why does the chess.com rating go up when you win a weaker opponent? Or the same thing. Why does it go down when you lose to a stronger opponent? 

Isn't that what is supposed to happen? losing to stronger and winning against weaker players?

Wouldn't it be better to just get higher rating when you win a stronger opponent and lose rating, when you lose to a weaker one?

Edit: or would everyone just stop  playing agains weaker players to not "risk" their rating

It depends on the difference in ratings. The bigger the difference, the less chance there is of an upset.  At some point-- I think it's about 400 points-- the high rated player gets nothing from winning while still risking points in the event of a loss. And the same is true in reverse.

Still, significant upsets do happen and swiss system pairings often force those matches in early rounds. 

The rating is higher doesn't you win, it's just mean you have higher chance of winning. So when a higher player win less and lose more when play a lower rated player. That basically mean, if the players are at their true rating, when they play each other a thousand games, their rating wouldn't change much. With your reasoning, if 2 players play each other, their rating will eventually become equal no matter how one is better than other. If you apply it to the rating system, everyone rating will eventually be approximately equal

I presume the OP was wondering why a stronger player should gain rating points (at all) for winning a lower rated player (or lose rating points for losing against higher rated players). And I also presume that he meant that ratings should change only if an upset happens.

Technically, "weaker" and "stronger" are subjective. If a 1600 player wins against a 1599 player, should the 1600 rated player gain any rating points since he is "stronger" than the 1599 by default and he is the player expected to win the game?

Even in game play, anything can happen - the weaker player could be having a dream run while the stronger player could have woken up from the wrong side of a bed.

Because of this uncertainty which exists, it is logical that a lower rated player has a chance of beating the higher rated player. To compensate for this possible upset, the higher rated player is given a small rating incentive to win the lower rated player, though this rating incentive decreases as the rating gap widens.

P.S. Just a random thought of mine. Imagine if the higher rated player did not win any rating for winning a weaker player, but loses rating for winning a stronger player. Where will the rating for the stronger player come from previously if this system had been in place all along? Supposing this site contains only two players joining at a 1200 rating. Player A wins player B, and hence Player A gets to 1300 rating and Player B gets to 1100 rating. Then. player A would be "stronger" than Player B and therefore player A should not win any more points for winning Player B. If this were to happen, then none of the players can exceed 1300 rating points.