You're not entirely wrong Johnny, but didn't Pete out and out DENY it for 15 years whereas Mark has just declined to comment?
Yes, Pete did deny it. And every reporter, broadcaster, etc. covering the game said, "if Pete would just admit his wrongdoing, we would all forgive him." Then, when he admitted it, everyone did a heel-turn and said "you call that an apology? After LYING to us all those years? Unforgivable!"
That kind of vindictiveness demonstrated by the media and fans would lead any ballplayer, including McGwire, to conclude that it's better just to keep your mouth shut.