...In Enterprise's case UPN and the Studio were same company...
Not really.
Paramount created UPN, but Paramount was then bought by the company that owned the TV network CBS.
CBS was given control of the UPN network, and so Paramount Pictures lost control of the
United
Paramount
Network.
(This is why when UPN later merged with the WB, the resulting network was called "The CW" rather than "The WP". CBS removed the Paramount name from the TV side entirely, and inserted their "C" in place of the "P".)
Paramount Pictures had nothing to say about the ending of Enterprise, because the powers that be at CBS cancelled the show. They were in charge of the network, and Paramount no longer was.
Also, CBS took ownership of all of Star Trek away from Paramount, and then Paramount was jettisoned off when CBS split into at least two separate companies. Paramount ended up owned by the new company that did not own Trek.