We've had them inspected by CBSA and they have no issues. In fact two out of three agents complimented us on them.
Since the base plate retainer is replaced by the block, the magazine would not function properly without it. If you remove the block and reinstall the base plate it would come loose as soon as you try to fire the rifle which would allow the spring and follower (and ammo) to drop out the bottom. I suppose one could reinstall the base plate with some epoxy or possible screw it on somehow, but this would require tools, plus the spring will not stay properly aligned on the base plate without the retainer, so it would likely not function properly.
As an authorized RCMP Verifier, I see this as a more viable option than the rivet, especially when applied to the polymer magazines. If I don't care about the aesthetics of the magazine, it takes me all of about 90 seconds to remove the rivet from a PMAG in the field with my multi-tool, but would require some additional tools and/or parts to make one of ours functional if I remove the block.