Why not? Maybe people already have an account there with payment set up, the console hooked up to the TV and soundbar and don't want do sign up somewhere else?
Furthermore, Sony Pictures is huge, so selling movies is absolutely part of Sony's business as a whole.
Who would buy movies use a TV subscription service from a laptop computer manufacturer? Or a company that sells telephones? What, are they watching movies on their old Western Electric Model 102? That'd be ridiculous.
Furthermore, Sony Pictures is huge, so selling movies is absolutely part of Sony's business as a whole.