I don't see how us signing Postma to a contract makes any difference on whether or not he makes the team this year.
He was an RFA and needed a contract, period. We weren't going to let him walk, so he was always going to get a contract.
As for the 1 way vs. 2 way angle, I'd imagine he's getting a 1 way due to 2 factors, if he makes the team he's not going to get sent down as he would never clear. So in that vein a 2 way contract makes no sense. Second, he might have been given a 1 way contract as a reward for his play, and as a good will gesture from the org.
As far as being sent down to St. John's, we all know he wouldn't clear, but I believe that teams can send someone down if it's X days before the end of training camp (don't remember what X is). So the Jets could still get him to St. Johns if he's sent down before that point, and he would be rewarded with the 500k. I don't believe he would be exposed if he was then brought up at some point in the season (correct me if I'm wrong), but once up, he would be staying up.
I love Postma's offensive skills, particularly on the power play. I think he can be a real asset there. However, even with his defensive zone deficiencies noted, I just fear he is redundant on a team that already has Buff, Enstrom and Bogosian, the latter of which who should see an increased offensive role, in my opinion. Postma's also right handed (to the best of my knowledge), which we seem to have quite the collection of.
...being -14 on a good Ice Caps team also isn't too inspiring. He's just not my favourite bun in the oven.