Just read the FAQ ruling on Djinn which states that it cannot be used to host Parasite. This does not make sense to me from an IRL perspective and a game perspective.

IRL (with the appropriate caveats for a fictional setting!) - If Parasite is indeed "hosted" on the ICE rather than somewhere else, why does it normally count against the Runner's MU?

Game - Without including Parasite, there simply aren't enough Virus programs (at this point in time), to justify playing Djinn over generic memory boosters. Though I guess this may change in future (and has indeed moved in that direction with the release of Imp).

There is no reason why this cannot be ruled the other way, as I believe it should.

I have a reason…

"A card or counter can only be hosted in one place at a time.
Parasite has the restriction that it must be installed on a piece of
ice, and therefore it cannot also be hosted by Djinn."

Glad I could help clear things up. ^.^

Damocles gives the rules reason. I'll hazard a guess about the game balance reason behind the rule. Parasite is a big annoyance to the corp: they have to either sacrifice the piece of ice it's installed on or regularly scrub virus counters (losing entire turns) and there is always at least 1 virus counter on it during the runner's turn. Without the hard memory cost, Parasite would be too powerful.

Parasite is hosted on the ICE, because it is applying the effect to the ICE.

Parasite is possibly one of the most powerful Anarch cards - and without having it hosted on the ICE (and therefore allowing the corp to trash ICE and Parasite) there would be few methods of handling it.

Djinn is a great card, alongside it's hosting it can tutor for Viruses.

Djinn is godly, Lets you Tutor viruses and host Medium. Datasucker, Imp and even Magnum Opus which is a surprisingly good economy card for Anarch

There is no way you are going to get enough MU for all of them normally

The rules also state that programs installed normally count against the Runner's MU. It doesn't matter if that card is hosted somewhere else; it counts against MU (this is why Parasite does).

Djinn, of course, has text specifically circumventing this rule.