You're fine.

As someone else mentioned, car electrical systems are highly variable in voltage; any device made to work in cars will handle 15V no problem (if that wasn't the case, the engineers who designed the system wouldn't be so stupid as to use standard automobile power ports, would they?).

Also, your "75W" inverter won't draw that much, unless the unit it's powering is drawing alot of power. I don't think a tablet will draw anyway near that much.

As for it being "dangerous", I don't see how. Note the power ports are current limited, such that they shut down if you try to draw too much power from them. 'found this out the hard way: I tried to share a single power port with someone else, using a Y-adapter. Each of our PC's drew about 50-60W. It wouldn't work; the port shut off with both of them connected. We had to alternate one-at-a-time.