If the fan motor was pulling too many amps, the fuse should blow.
The reason the fuse holder melts is because the fuse contacts rust and/or corrode, creating resistance which creates heat, thus melting the solder on the ends of the fuse caps. The reason other fuses don't melt this way is because no other circuit in that fuse box draws that much current. Heat buildup because of resistance is in direct proportion to current flow.
I repair this by yanking the two wires from the back of the fuse box and connecting them to a 30-amp circuit breaker.