There absolutely should be a legal clarification of what "buy" means with respect to digital goods. And then fine everyone who uses "buy" instead of "license" for false advertising.
Rent has a defined time period. Buying a license in this case has an indeterminate time period. Buying a license is still very different from renting or leasing it.
So, a rental agreement for a defined period (very short, instantaneous even), with automatic renewal.