For one, Evgeni Nabakov has established himself as the Isles starter this season. For another, Thomas has no plans to return to the NHL, according to TSN's Bob McKenzie.
But the move still helps the Islanders because it "gives us roster flexibility," GM Garth Snow said, per Newsday's Arthur Staple. The Islanders were already over the $44.3-million salary cap floor, but adding Thomas' $5 million cap hit to their books (New York won't actually have to pay the former Boston Bruins backstop, who is on an indefinite leave of absence) would allow them to make trades later this season while staying above the minimum payroll.
It also benefits Boston, which gains the cap space Thomas's contract was eating up.