I was playing Point Lookout (again) and I noticed that all of the weapons from there (double-barreled shotgun, lever action rifle, axe, etc.) said -75 HP under affects (I tested it and it actually took away from my health). Is this a bug? I hope so.

Do you mean you hit an enemy with it and it took away from your health? It sounds like a bug to me, I don't remember that from point lookout, although its been awhile since I played it. The only weapon that I remember having a -75 HP affect on the player was the tri-beam laser rifle from broken steel.

Maybe that's why those dang tribals kicked that crap outta me in 2 shots...

It was the hardest DLC in the Bethesda/Obsidian Fallouts IMHO, so what do you expect?