The all have them, and they do not effect performance, at least nobodu has been able to tie any failures of any kind to them.
While I have not, nor will I ever mind you, taken an MP frame and cross sectioned it, the bow is a side effect of molding in the rail on the bottom of the dust cover. Just by looking at molded component ther appears to be thick spot in the material at the transition from the trigger guard and rail section. When the mold is filled and allowed to cool, the mass of material at this thick spot cools and shrinks differently from the rest of the part. In the case of the MP frame that differemce appears as the "bent" dust cover. The "bending" can be controlled by either modifying the part design and tool, changing the molding processes... All these add cost to the past either up front, or to each individual part as it comes off the tool due to increased cycle times and such. Given that the bow has no effect on performacne, but is simply an aesthetic thing, I guess SW chose to not mess with it.