I think Carol has it correct. They're looking the other way because it isn't worth forcing the issue. And if it does become an issue they still have legal recourse if necessary.
When Microsoft was attempting to establish Office as the de facto productivity suite they overlooked a great deal of the casual piracy taking place. Once Office was established; and WordPerfect, Lotus, Harvard Graphics, Smart, and other rivals were safely out of the way, Microsoft started getting serious about their licensing by introducing a much more real activation program that included Genuine Advantage. As some competitors pointed out back then, how can you compete when Office is effectively free?
I think a little of that is also in Adobe's strategy.