If you are refreeing to the USB stick that Apple includes with MB Airs that shipped with Lion, then the reason it won't work is that Apple specifically caters those towards whatever system they ship with. They work the same way the grey restore DVDs worked; I.e. You can't do a fresh install on a MacBook with discs that came with a Mac Mini.
If you are finding that Lion seems to install on other machines with the USB stick, it's probably because Apple didn't intentionally block all machines with it; just previous MBAs so people couldn't circumvent buying Lion on the app store.
If you are referring to a Lion USB that YOU made with a Lion dmg file, then not sure why it isn't working.