[joelennon]

While supporting SSO authentication is relatively trivial via SAML2, which is supported by virtually every identity provider solution in the market, configuring this is likely a frequent point of support contact and configuration problems.

Something that is typically much less trivial is user provisioning from an identity provider. Most companies availing of SSO will require an automated user provisioning process also be in place. While the SCIM standard is pretty good, and support for it is getting better, implementations of it in identity providers are quite varied and often incomplete, not to mention poorly documented. Companies often also need to provision data that is not stored in the identity provider, so you can imagine the challenge in providing alternative solutions to make that work.

As others have already pointed out, it’s far more likely however that vendors see the requirement for SSO as a signal that a company is larger and falls more in the “enterprise” tier of pricing, allowing them to instantly provide something of value at a higher pricing point. SSO is treated much the same as things like premium support and SLAs - you charge extra because the customers who want it can afford to pay for it.