This is the real reason there's an SSO tax. It costs to support SSO, the customers who want SSO should pay for that cost.
But this will usually show up in pricing structures with SSO as a relatively inexpensive add-on feature -- not as part of an indivisible bundle.
Seemingly we want to talk about both of these things as "the SSO tax" but I think we can agree that they're pretty different scenarios.
SSO issues usually takes 10 or 20 times longer to sort out any issue.