There are a broad range of ways that they could apply an interpretation for commercial activity, including the intent to make a profit and the actions to promote the offering for rental.
The timeshare companies only have the patterns of use data to base any actions on, so its easier for them to put the criteria in that you describe. They don't even have the information on whether a particular reservation was rented out or gifted, just that it is being used by someone that isn't the owner who booked it.
Unfortunately that means they can only put in restrictions that capture people who are not renting out as well as those that do. IMO that's why we are seeing a nibbling away at the edges by MVC, to try and find the way to discourage those who are abusing the reservation system without causing disruption to legitimate usage.
It will be interesting to see how this evolves.....
Perhaps they will improve their offering under their rental programme to encourage more volume that way if the scale of the external rental market is enough to be worth it.