It's clearly tied to their cinematic decisions and restrictions.
It'd make no sense if they weren't mutants, though, given the events post-M-Day.
Quicksilver was depowered, and got wonky powers via the Terrigen Mists. Especially if he'd already been an inhuman, this just seems off.
If they want the twins to not be Magneto's kids, I don't know but there COULD be a decent story there. Making them not mutants seems forced at this point. But then, it's not like continuity following Magneto and company has ever been intelligible.