In my educated opinion here…
PragerU really has no grounds. While they base this on the Pruneyard Rule, that rule is very narrowly defined. And over the years it has become even more narrow by the application of the first amendment of he US Constitution, rights afforded by varying state constitutions and the Communications Decency Act, specifically section 203.
All one needs to do is take a look at the original court decision of Robins v Pruneyard, then a variety of other cases and rulings and you will easily see how PragerU is barking up a tree of claiming foul on free speech by attacking said free speech and it will not hold.
If you want to dive into the sources that form my opinion here, check out Robins v Pruneyard, Ralph’s Grocery v United Food, Barr v Camelot Forest, Shulman v Facebook, Fashion Valley v National Labor Relations, Med Lab v ABC, Aldrich v Times Mirror, Zhang v Baidu, Fairhouse v Roommates, Zeran v Am Online, Donahue v Creation Outreach and many others.