How is hacking achievements any different than farming them instead of earning them? I played through campaign, but I don't remember any (non-hidden) achievements that were particularly difficult but my point, however, is that people who want to "cheat" for their achievements are going to do it anyway, whether through a hack or through cheap/repetitive gameplay.
Good example was when TF2 added some achivements awhile back. First people just turned most of the servers into achievement-farming games, then achievement servers showed up, and then the text-editor unlock showed up. In all cases, they are getting achievements without actually "earning" them. You ban someone for hacking achievements, then they'll just farm them (which is basically undetectable). Integrity of the achievements is no longer valid in either case, so why ban people for single-player hacks, since it just makes the community upset and does nothing about the perceived problem.