The truck is trying to brute force move the Mjolnir because the owner is trying to move it.
The Helicarrier isn't trying to move Mjolnir. It moved with Thor and it would be rather silly for Thor to board a craft and not have it move along with that craft. Note that when Hulk tries to move Mjolnir, it doesn't move for him.
Thor placed Mjolnir there. We don't ever see Mjolnir be heavy, just immobile should someone "unworthy" try to move it. I would assumed that if Hulk came in, he'd be unable to remove it from the hatrack.
Ultimately, the implication of the statement on the hammer is that if someone desires to move/wield Mjolnir, and it deems them unworthy, they (and anything they use to that end, like a truck)will fail.
If a worthy person moves it onto something mobile, it seems it will stay on that craft until someone worthy removes it, regardless of where that craft would travel.