The very fact that the players can only throw Innova discs and not other brands, I'd say there is a continuity of relationship there for sure. If the players were indy contractors, they would be allowed to wear and throw whatever other brand(s) they wanted to.
I bet they're not considered employees.
They're free to set their own hours, choose their own tournaments, get paid (mostly) by others, throw discs they own (even if given by Innova), and aren't actually doing labor for the company. They're advertising by affiliating with its products. They're also free to strike up endorsement deals with non-disc-manufacturers, such as a shoe company.
And I'll bet Innova's attorneys have structured the contracts to make sure the sponsored players are not employees.