Part of me wants the Mac market share to grow so there'll be more Mac games written while the other part dreads the consequences of this.
1) Broken Windows Theory - Windows is like a bad neighborhood - such things are accepted. Mac users don't tolerate anything that doesn't "just work" If someone wrote a piece of mac software that installed this crap, it would be all over the web by the afternoon, and that person would never be able to write a piece of mac software again.
2) Better Security to begin with - Network services are off and ports closed by default. In addition, there are fewer places for crapware to hide. If it tried to install itself, it couldn't do it without authorization.