Thank's Zoichan. I feel like I'm being extremely dense on this issue. I was under the impression that it was another container that auto-poisoned you without player interference. Good to know it's just the water pack the complaint is about.
Thieves and ninjas can envenom other containers as well, not just the pack, and I gathered the pack was preference for a larger number of drinks per container. I'm fairly certain that having Farmer John's water barrel envenomed will have the same effect. Same with a light water jug from Elemental Canyon. Or a bought container from Maelbrim. If they don't, yes there's a bug.
If it does, then the "specific container" was the source of my confusion and this whole issue revolves around the magic resistant races getting a higher level poison from thieves. Like everyone else can. But everyone else can use brews, too and magic resist races can't. The magic resists had to go out of their way to get something that would poison them. So the problem lay with envenoming containers. But that's a class feature. If one were to make it harder for magic resist races to get poisoned from envenomed containers, one could lower the poison level on containers from thieves and ninjas. Perhaps.
If it's not the envenom skill, the underlying issue seems to be the want or need to have a counter to paralytic/catatonic skills without resorting to poison. I'm going to bow out as I've got nothing to bring to the table on that issue, or on the original suggestion in this thread.
In fact, I think I'll do that anyway as I feel like someone needs to explain this to me like I was five.