The verb being used here is “bound”, not “bind”, as @Aeruniel said. It refers to the reduction in your max health that is the “cost” of getting the +8 attack.
As a native English speaker for probably much longer than you, as well as a professional writer, it makes perfect sense to me. Never once did it ever occur to me that it meant anything other than what it sounds like: the past tense of “bound”. Likewise, I have never heard anyone else ever complain about this before, and most players of this game are native English speakers.
It’s pretty normal, I think, for certain words to just “seem wrong” to our brains even when they are fine. It happens to me on occasion. I think this one is on you buddy. 
With only the small disadvantage of actually meaning the exact opposite of their intentions. 
PS. Google “bounded down the stairs” for another use of “bound”.