Right, it's not like their cost of production is proportional to the number of computers the software is used in. It's like trying to buy a vacuum cleaner and being charged per room you plan to use it in.
Whatever both buyer and seller agree on. I think in general buyers can agree that, by the nature of software and the existence of the internet, if people share their bought software like they would any other object, then the seller risks not being able to make any profit at all. Since sharing software is copying software, there's no equivalent of "returning" what was borrowed. It's a compromise most buyers can understand.
However, charging per machine of the same user seems to cross the line, no? Sellers are pushing, and I think it's valid here for buyers to push back.
EDIT: Could we have the courtesy of including explanatory replies with downvotes? Simply downvoting seems like the equivalent of replying, "No. Shut up."