They can, and I suppose the question is whether the border checks that the OP is talking about are "sporadic" or "systematic". There was a time, a few years ago, when it was not uncommon for France to do random-ish checks on people travelling by car from the north (?thinking of Amsterdam and the Netherlands), for example. I don't know if these checks were within the letter or the spirit of the law. Border checks are sometimes carried out in the run-up to a major event, such as a sporting final or a G8 or G20 meeting. Such checks are certainly permitted.