They have a bin near the gates and check in desk that is the exact size requirement and final determiner - if it fits into the bin, you can bring it on. It also does not apply to any tickets purchased on Frontier's web site - those include a standard carry-on free of charge.

I'm not sure what is deceptive about it - they spell it out on not just one, but two separate tabs at the top of their website ("Optional Fees" and "Bag Fee Changes"). Can you please clarify that point?