Because the engineers of the other software chose not to work for free?
I'm not saying they shouldn't be compensated, just that justifying the SSO tax due to the support burden is pearl clutching.
Do you mean that the support burden is not a valid reason to charge for SSO?
In my experience support us the most tedious, annoying, and time-costly part of building a SaaS so I would definitely always charge extra for anything that requires more support