Hi Scott
As far as I am aware the Yamaha part is the same as the Akai and VST sections of OAS, in that you can play them so long as the manufactures trademark is advertised. (VST is the Trademark of Steinberg GMBH etc)
As to the Tyros 2 styles these are not supplied by Wersi, (Only the ability to play them or create your own) but by the dealers who have most likely downloaded them from the web and put them on a disc. (All the Tyros 2 styles are freely available on the web)
The OAA itself (Not just the Yamaha section) has been under development for about 2 years, so I am sure Wersi will have checked that it is legal. (If you were a small company like Wersi, would you want to take on a giant such as Yamaha?)
In fact if you think about it, if it was illegal then so is all the conversion software that is out there, not forgetting OMB which also uses the Yamaha style system.
BTW If you can read German the manual can be downloaded here. (There are plenty of pictures) http://www.wersi.net/Handbuch_OA-A_1.Aufl._2007.01.16.pdf and the brochure here http://www.wersi.net/INFOBLATT_OAA.pdf

Bill