Not unusual
Several prominent hosting companies have sold people deals where the hosting company owns the domain name making it impossible for people to move to another host. I'm surprised that after years and years of there being the Web, businesses are still naive when it comes to IT ownership. Companies such as Square Space and GoDaddy will box people in by getting them to build web sites that rely on proprietary tools to create web sites that can't be transferred to another host. Yes, the tools are quite nice and many of the ready to go templates are very attractive and have many functions, but when the next year's renewal comes at 3x the price, you either have to pay up or spend time rebuilding your whole site someplace else.
It's all as legal as church on Sunday, so unless there is gross negligence or provable failure to uphold contractual terms, the dental group may be out of luck. Even if they can get some sort of relief, the domains might still wind up in the other company's possession. In the mean time, the sites going dark will mean people deleting their bookmarks and search engine de-listing.