This is not IDB. That happens when you are denied boarding on a flight that leaves due to an oversold, or some other, consideration. This is a simple mechanical issue. The usual procedure is accommodation on the next flight and overnight accommodations, if required. In this case, they gave you a shuttle bus.

I think that the $75 was fair, all things considered. If you complain, you may get some miles. You may even get the $75 re-offered. But do not use IDB when you do as this does not apply.

Cheers.