I don't know about the very beginning, but at least since 2013 Tesla has publicized the rule that Supercharger stalls should be freed after the charge is complete, so I think it is perfectly fair for them to ask that from there onwards.
Was it in a contract though? You've said before what Tesla said publicly is good enough. Tesla also indicated several times and at Supercharging launch event that the intent of the network was to facilitate long distance travel so if we should follow vacating the Supercharging stall when done charging we should also do that as well.