Yes that would indeed be meant to happen happen unless you locked off all OTHER joints so maya knows where to put/ take the rest of the weight from. See there just has to be weight and the extra wight has to go somewhere. No if you paint one joint black the data will go to all the other nonlocked joints equally distributed.
Wieghts do not disappear they stay constant, the wight is shared amongst all the remainding joints. This is why its often not so productive to erase weights just add as its more predictable the thing what got taken away then what got put in. Then flood one joints with everything and start adding away in others.
Incidentally you want to use rgid binding here, as you dont use fractional weights.
