The rate on our HELOC's automatically accelerates to 18% when the loan matures.

I am being told that this is okay legally, per contract law, we can accelrate the rate in the event of default (in this case, not paying it off at maturity).

Does anyone know if there's a link or statute somewhere I can refer to that confirms this?

This is going to be totally based on State law. You might try the FL forum.