Is it legal for cox to change commercials?

We've had Cox communications cable for the last year and I've noticed that many times networks will be playing their advertisers designated ads, however then Cox will insert their own local commercials in, not even waiting for the current commercial to end. This is extremely annoying when they decide to insert commercials during TV shows (even the Superbowl). I was just wondering on the legal aspect of this and if this is normal for TV providers to do.