Can someone explain the mechanics for why Cursed Ground is not purgeable?

[[ZZD]Habile]

^ I didnt realise this i stand corrected

There are many useful threads in the Guides and Lists section of Mechanics. I'd recommend browsing them; they tend to be rather interesting if you're a fan of mechanics.

There's also much to be found in the code and scripts themselves. ElementUser posted the contents of the base.gamemechanics file, which is quite relevent to some of the topics in this thread.
http://forums.heroesofnewerth.com/sh...ad.php?t=55347

[[BRO]Mellow]

Yes. If I remember correctly (I could easily be wrong), it was this way in DotA because the spell was far too complex and thus was handled 100% by triggers. Because of this, it lacked and still lacks a status debuff icon entirely. As most people have said, it has carried over like it is from DotA, just that the HoN engine is flexible enough to provide us with a little status icon for it.

I assumed this too
I also assume this is the reason why Demented shaman's slow isn't purgable either.

[[ZZD]Habile]

I assumed this too
I also assume this is the reason why Demented shaman's slow isn't purgable either.

Purgable, actually. At least in DotA.

In HoN it's classified as a StatusDisable, the same as stuns, which are not removable. The poison damage effect doesn't have any type associated with it, and thus not purgable either.