Polanski's lawyers at the time should have (and most probably did if they're worth anything) made it clear that a plea bargain is not set in stone until the actual hearing when the judge makes his final ruling and sentencing. The judge, right or wrong, changed his mind. Polanski's back up plan in case of such an eventuality was obviously to flea and become a fugitive. Great plan, unless you get stupid and careless. He got stupid and careless.
Choice -------> Consequence
Personally, I think that the matter should have been dropped long ago. If he ever voluntarily set foot on US soil, then, yes, by all means, grab the repulsive individual and deal with him through the US courts. But this whole international man-hunt over a 30 year old questionable statutory rape case was just not necessary.