Well, to be a bit fair, there ARE interfaces that have HZ inputs deliberately designed to add "colour/attitude/warmf" whatever you want to call it (I calls it all DISTORTION!) and these often get praised in reviews. My view is, that is fine if the effect can be defeated, if not then I am not keen as I believe such an input should be as transparent as possible. After all the punter might want to plug in something that is NOT a guitar or bass!
However, the vast majority of interfaces have a simple, clean Instru' input and if the specification says it is 1 meg Ohm the likelyhood is that is just a jfet op amp. This is probably why most AIs have at least two HZ inputs. Op amps come in pairs as a rule.
There can be a problem is the AI gives the HZ input too much gain. The temptation seems to be to enable it to hit 0dBFS when one just breathes on a feeble Strat clone. In fact you only need enough gain to get about -20dBFS with modest strumming. IMHO the front end of an AI is NOT the place to add distortion.
Dave.