No, it's just simple and good price discrimination.

Don't think of SSO as a "tax" you have to pay on top of what is otherwise a cheap and fair base price.

Rather, think of the enterprise price (which includes SSO) as the fair base price that allows the vendor to stay in business and pay their costs.

While everything else (non-enterprise) is severely discounted, in the hopes that if you do become enterprise you'll pay full price then.

And you need something to prevent enterprises who can obviously afford to pay, from buying 1000's of seats at discounted rates. So you discriminate based on something they absolutely need: SSO.

It works really well.