Hissing is imho not always aggression or defensiveness.
Its simply an expression. They do not have to many, and that is one of them. I never worry about hissing, unless its in conjunction with an upraised upper body or neck, head facing me, staring me down, tight s-curve. Then I may think, hm, I may get tagged.
A hissing while getting disturbed is akin to us saying "aaaaaawwwwww man" while someone shakes us to wake us up, and we don't feel like getting up. I don't even pay attention to it. They certainly have a right to voice their opinion, LOL.
I've been bitten 4 times since owning snakes (never by Ball Pythons). 2 out of them were nothing. Tiny Green Tree Python hatchlings that were taking a pop at me. Didn't even feel it, even though it left tiny marks. One was a carpet python. I felt that one. But again, no big deal. The 4th one Redtail Boa. OUCH. Definitely felt that one!!!
Neither of those hissed. Struck and bit faster then a speeding bullet
A bit of complaining doesn't bother me at all. I find it sort of cute when they talk back a bit