Generally, a 9month Tenured isn't obligated to take summer classes without compensation. Also, typically a Tenured would not take more than two consecutive summer classes. It tilts towards illegality.
However, it is not uncommon for some universities or school or department to include a clause in their policy along the line of ... Summer classes will be assigned based on the best interests of the university/school/dept. What is 'best interest' is left imagining!
PS: even at that, not that the Tenured can just be bamboozled or compelled with recourse.
Kindly check your institution policy and take it from there. If you have union, get feedback from there: they might want to take it up for you.
If your policy has a resolution clause, explore the route provided.
If you're being singled for 'no pay', raised it firmly with facts yet respectfully. It might sit on #discrimination leg.
[Edit] Might be worthwhile scanning through the #reddit post The legality of Requiring Work Over Summer for 9 Month Contracts.
NB: any advice/comment bothering on law on a faceless forum shouldn't be construed as legal advice.