Razors are somewhat a needle-like object, and it is inevitable that during shaving blood will get onto the
razor. Someone who shaves, using a used
razor (of someone who is HIV positive), may get cuts while they are shaving that may possibly allow the virus a way to enter into the body. It's probably unlikely, in the first place, that the virus would even survive on a razor... but there is a bit of risk, so why even have that risk when it can be completely avoided?