The fact that they can get away with continuing to serve Americans for 270 days or maybe indefinitely doesn't matter.
The 270 day provision is for the implementation of banking rules. More to this point is that if online poker is made illegal by this bill (a highly disputed issue), then it happens when the bill is signed. A relevant short term issue for Party is that it has business arrangements (lines of credit with British banks, etc.) that are explicitly and contractually contingent on the legality of online poker in the US. These are issues that do not immediately affect smaller, privately held sites.
Quote: My guess is that as a British public company that feel it is important to maintain, shall we say, their DIGNITY. Perhaps not the perfect word but close enough.
I think that what you are getting at here is maintaining a reputation as a cooperative and law-abiding major player in the US economy. Online poker will eventually be legal, regulated, and taxed in the US. When that happens, those sites that continued to operate contrary to the spirit of the law will not be looked upon favorably by the regulators. (Like employees who sue over wrongful termination and get their jobs back by court order.) Party will be welcomed back like the prodigal son.