This is absolutely true and has been U.S. federal law for some time, basically saying a manu can't force you to buy their oil, parts, and/or other maintenance items.....
However, manu's are getting very smart at adopting new "standards" that they expect oils, etc. to meet (read: JASO and other, newer standards) that they expect would be used in their new engines. It then takes time and $$ for aftermarket oil companies to get those new certifications. Failure to use an oil meeting some new standard can result in a declined warranty claim should there be a defect. And warranty companys have been known to collect oil samples out of the cranks of sleds in for engine warranty work. Guys have run into this using some very good oils in their Polaris 800 CFI motors. Just an FYI is all. Any corporation is capable of treachery, especially is the powersports warranty industry. In my RMK, I run Polaris VES II Gold for that reason.
In my Attak I run Amsoil 0w40 and it's great. In my Mt Viper I ran Amsoil Interceptor. Amsoil is no good in Polaris 2 strokers because while they are good, Amsoil doesn't have the correct certifications.