why do you think it should be ok to prosecute cases involving HIV, but not with Syphilis or Herpes (both life long incurable conditions)?

Because HIV is a deadly disease and the other 2 are not. (Syphilis is a disease that nowadays gets cured with antibiotics and herpes gives you the itchy sore every now and then that goes away with a couple of pills. A lot of people with herpes are  even asymptomatic).

Fortunately  HIV is becoming "less" of a deadly disease thanks to the advance of medicine, but since the 70's  people have been dying of AIDS (about 30,000 million reported by the foundation of AIDS research and 1,7 million in 2011). Presently there is currently no cure for AIDS or HIV infection. Although antiretroviral treatment has been improved greatly in the last 5 years suppressing HIV ? the virus that causes AIDS ? and can delay illness for many years,but still  it cannot clear the virus.

In the future once a cure for HIV is found it will become "just another "STD", the laws will change and  I'm sure "criminal transmission of HIV" will disappear from the legislation.