The answer: It's prohibited by the CBA:
A Player who has been acquired by Waiver claim shall not be Traded to another Club until the termination of Playoffs of the season in which he was acquired unless he is first offered on the same terms to the Club(s) that entered a claim when Waivers were requested originally and the offer has been refused.
Basically, if both the Isles and Rangers claimed him, the Isles would have to offer him to the Rangers on the waiver terms. Of course the Isles could have claimed him, keeping him from the Rangers, and kept him until the end of the season and then traded him... but this wasn't what they did.
The Rangers, btw, already have over $41 million of payroll committed for next season to cover 9 players, with Antropov, Morris, and Zherdev all scheduled to become UFA's at the end of the season.
Forever1940 is the nom de plume of Eric Hornick, statistician on Islander home telecasts since 1982.