The accelerator sounds scummy, but at the same time i can't help but wonder wtf the owners of these companies were doing. Did they just not read the contract? If you own a company i think you have a lot of responsibility for the shitty business deals you make. Its not like we are talking about some senior citizen hoodwinked into signing their home away.
That is generally how contracts work. You get something and you give something in return. Nothing comes free. If the deal was better then "the highway", you have no cause to complain when the other side comes to collect their part.
Especially for a sophisticated party like a company. Things are a little different for individuals like employees where the power imbalance is coercive. However when it comes to a company, as long as it wasn't outright fraud, i have very little sympathy that they are having buyers remorse over a bad deal.