Heard James Earl Jones interviewed on Radio 5Live in the UK while driving through town one rainy Friday afternoon a couple of years ago, and he was asked about why his filmography contained so many bad movies, and his answer was essentially that, in spite of his (poorly paid, as he only provided the voice) fame as Darth Vader, he is still just a jobbing actor. He doesn't get paid millions per movie, and sometimes accepts what he knows are poor scripts because he needs the job and the paycheck.
I also once saw it written about Christopher Lee that he was "the finest actor ever to appear in this much crap" ... presumably he had the same need to accept roles that paid the bills.
Appearing in an Uwe Boll movie would seem to go way beyond that, though.