They have to be more accommodating than you think for various reasons including...
- some transponders came with faulty batteries
- occasionally the signal just isn't picked up
We have EZ-Pass (which also now ties in w/ I-Pass IIRC) and every once in a while I'll get a fine notice for supposedly using EZ-Pass lanes "illegally" even though transponder was definitely on the windshield. Of course if they just cross referenced the plate w/ registered addresses of transponder owners they'd see I have 3. Instead they mail a fine + toll due notice (probably hoping you pay). All you have to do is contest the fine by filling in your transponder #s and mail it back. They then send you an apology letter and deduct the toll due from the acct.
Ultimately, it's a little game of liar's poker and the private companies that run EZ & I-Pass are most likely hoping to get some add'l revenue. They know the system isn't perfect and thus must accommodate registered users as such.
So........
- contest the fines
- make sure all your cars (if they outnumber the transponders you have) are registered
- state you know the transponder was mounted as prescribed when you went thru
