This feels largely pointsless. 99.999% of services don’t allow account sharing, so this point would just be cluttering-up the point-list IMO. There’s very little services which do allow account sharing.
TL;DR I don’t think we need that case. And if it was the other way around “This service does allow account sharing” it should be negative.
From the point of view of a service provider account sharing opens me up to support requests and sometimes even liabilities when I would allow account sharing.
For example I might have to work together with authorities if criminal behaviour was conducted with an account. Or I have to proof the customer that they ordered an upgrade/product, which is harder if multiple people have the same permissions without username-separation (and I officially had allowed that usage).
In addition sharing accounts often means sharing passwords, which is a security risk in itself and something every portal should forbid.
The correct implementation on the provider side would be to allow a number of concurrent users (but with separate login credentials), which is hard to implement and not necessary for most applications.
This is why many services (including many I was involved in developing) officially forbid account sharing, but don’t enforce that rule if no problems do occur.
To be honest, if we had “This service does allow account sharing.” as a case, then I would classify that as negative, automatically assuming that the service provider doesn’t care about security.