Unlimited re-raising heads up (limit question).

[Shoe]

The B&M's by me have the rule where you can raise and unlimited number of times back and forth when you are heads up. However, this rule only applies if that round of betting started heads up.

For example, if there were 3 people going to the river, and 1 of them folded on the river, the other 2 would not be allowed to raise unlimited because there were more than 2 players that saw the river. My opinion is that once it gets heads up (even if 8 people started the round), the remaining 2 players should be able to raise back and forth as much as they want, as either play could still end the betting at anytime by calling. Why should having someone fold that round prevent unlimited raising heads up?

This rule does not make any sense to me, and I'm just wondering if there are any reasons for it, and if most casinos operate this way or if it is just a weird rule by me? Thanks.