There is nobody who has more DNA samples on mutants than Sinister. Stealing DNA samples is his whole thing. Saying Dr nem or Beast would have as much DNA samples as Sinister is flat out insulting.
Cutting deals with former enemies for the purpose of personal gain is something human nations have done for centuries. Yet suddenly it's a problem when mutants do it for the purpose of giving mutants a leg up on the rest of humanity.....
Magneto already spelled it out. "we are going to use humans own tricks against them."