You are making a correlation that is completely install dependent. One does not always go hand in hand with the other.
Your example only holds true if the amps is making the same power at both the higher voltage (low amperage) and lower voltage (high amperage). This is most likely not the case.
Say you are running a stock 100 amp alt. Your wire is fused at 300 amps. Your amp is capable of pulling 200 amps. You will sustain voltage drop without even getting close to the fuse rating (without even taking into consideration my other point below).
And further, fuses are NOTORIOUSLY crappy at blowing at their rating. Most fuses will let through 5x their rating without breaking a sweat. Thus, your 300 amp fuse just turned into a 1000 amp fuse and good luck trying to pop one of those things because of your "low voltage" situation.
T-fade is correct. The fuse is there to protect the wire and car, not your amp.