You're basically right. If the fuse is 7.5a, then the wires can handle at least that.

Typically automative wiring is a bit oversized, because you can't spec an exact wire size for a circuit's draw. You have to buy the size that supports it, and that may be a bit bigger. For instance, to run 15' of wire to support your 7.5a draw, you'd use 12ga wire. But that wire can handle 10 amps, so you have plenty of reserve.

And the fuses are also a bit oversized, because nobody wants a car that'll constantly pop fuses. The device on that circuit might draw 5 amps or less normally.

Dashcams draw very little power. Google says 0.5a at 5v is typical.

The result is that you have a healthy safety margin built in, and you're not risking much by plugging a dashcam into the circuit.

Empirically, I've done it on 4 or 5 cars at this point, and have never had an issue.