Sure, and with hundreds of those places, and at least 4 volunteers per place, just how many votes do you think you can stuff? Plus any statistically significant deviation will be spotted. But besides theory we never had more then the odd case affecting a handful of votes every handful of elections for almost 80 years, so history proofs that for all practical reasons it is 1) not happening 2) impossible to do at a scale that would impact results and 3) easy to spot.