Hi Stefan, yes unfortunately that's a limitation of simple bezier splines  there isn't really any way to get a fully exact circle with them. That's one of the reasons why NURBS curves were invented which are sort of built on top of some of the same concepts as Beziers but with additional capabilities one of which is the ability to have shapes that are completely exact conic sections.
So the circles that you are importing are not true circles at all, they are just a general squishy curve that is fairly close to resembling a circle but off by some amount from a true circle like say 0.02 units of deviation or something like that.
MoI's center snap will only snap on to an exact circle or also a general curve which is very tightly close to being a circle (within about 0.000001 units of being a circle), these curves you are dealing with must have much more error than that.
There is not any built in way in MoI to take nearly circular curves and build exact circles from them. But you could do something like draw a new exact circle using circle 3pt and snap the 3 points onto spots on the approximate circle to get an exact one.
 Michael
