They won't though....that's the thing with Century, its a bit of a crapshoot when you buy one of their rifles. Mine is great - solid and reliable, but I had to do some tweaking to get it right (sights and bolt gap). Also replaced the furniture for G3 stuff.
AFAIK Century do ship some Cetme's with a G3 mag - its just luck of the draw. As nalioth has pointed out, the receiver is patterned on a G3 well anyway. But at under a dollar per mag, its no big deal. Get some, then get it to the range and see how it shoots (after you check your bolt gap).