Why souldn't people with HIV get the shingle vaccination?

Live virus. This is a live virus vaccine. That means that you get a very mild infection, mount an immune response and are then protected from the real thing. Folks with HIV are unable to mount a vigorous response to the vaccine, causing bad things and not getting the immunity. The vaccine not only doesn't work for them, but they might also get chicken pox or shingles from it.
May cause infection. Hiv infected patient are immunodeficient. Shingle vaccine made of live attenuated virus which can cause disseminated infection in immunodeficient patient.