Why wouldn't they move him? After trying to get him to come over he responded by signing a KHL contract that expires the same year that the Islanders' rights to him expire. That says a lot about his intentions to come over for the islanders. Throw in that the Islanders have other interesting goalie prospects, and why wouldn't they get something for him before he can sign as a UFA wherever he wants?
Sorokin was in Colorado on a boat fishing with Varlamov when his extension was authorized I believe. Also, there was some "power of agent" involved in that extension, so I'm not sure if Sorokin intentionally signed that deal with his NHL rights in mind. It was likely that he set parameters for a contract he would willing to sign, and his agent (trying to maximize for their own good) negotiated the biggest and longest deal possible.
I think Sorokin would be willing to buy out the remainder of his contract to come play for the
Islanders, however not in the AHL. With Halak and Greiss under contract for this season, Sorokin was not coming over this season regardless.
Also, the Islanders only really have one other goalie prospect by the name of Linus Soderstrom. He signed with NYI this past season, and was loaned out to his SHL team, but also recently had surgery. In addition, he has Aspergers and ADHD so there are a few question marks there while Sorokin has been excelling in the KHL and could be on the Olympic team.