The point of the damage cap?

It's good practice to use consistent typing; if HP was an unsigned 32 bit int and damage was a signed 32 bit int, you would have to do conversions every time you wanted to modify one variable using the other. Generally, it's best to stick to 32 bit ints unless you have a good reason to do otherwise.

Well power creep shows that signed 32 bit ints was a bad decision.