If people stick slavishly to the rhythm it's perhaps because that's the way they been taught or trained to play. The musical notation system we use is, in many ways, a straitjacket and I would suspect that most music teachers are unable to even remotely envisage taking liberties with the rhythm and/or the time signature. The very idea is probably unthinkable!

@JohnInKansas: The best ceilidh/barn dance/ folk dance bands in my opinion are those where the tunes have been learnt and where the musicians keep an eye on what's happening on the dance floor.