According to the Terms and Conditions of the site:
Since all the content we give to the site is under a cc by-sa 3.0 license, any copyist of this sort has two obligations:
Give proper attribution, and
Therefore you are free to yoink their solution as well, golf a few bytes out of it, and claim it as yours once again. Frontier justice!
According to a Higher Standard:
It's debatable whether we can enforce anything that contravenes the site terms, but we are certainly each free to hold ourselves to a higher standard. There are the rules of the game, and then there is the Olympic spirit.
Sportsmanship has never been about legalism. It's about encouraging the best in others by bringing the best of yourself. Tim has reminded me that I, at heart, repudiate the flippant wit of my first answer (seen above) and believe strongly in a spirit of fair play.
I have personally abandoned answers I was about to post because I felt I had nothing to offer but a couple of bytes reduced from an approach already submitted in the same language by someone else. That's not required by the rules, but it's required by my own sense of what is right. And I think the majority of the community shares that sense, because I see people frequently commenting to improve one another's answers. Or to ask permission to borrow part of an algorithm for their own answer.
An interesting thing happens to us as we age. As adults, we adopt a "play to win" mindset, but if you look at many of the games played by children, children often play to continue playing. No one wins at freeze tag or hide and seek. You simply continue the game.
Obviously the individual challenges here on the site have winners, and must do so because the accepted answer is a core principle of Stack Exchange. But beyond each individual game there's the meta-game of the community itself, and you can play that game to win, or you can play to continue playing.
Which, stated another way, is playing to continue the enjoyment of the community instead of merely enhancing your own enjoyment. And I'm glad that style of play is in ample supply here. It may not be universal, but it feels like the prevailing spirit.