According to MCPE-170535, it is intended design for negative integers to be accepted values for /scoreboard players add and /scoreboard players remove, where adding negatives would simply decrease from the current score, and removing negatives would increase. This, however, is not the case.
Additionally, this is inconsistent with /experience add (which has no corresponding remove argument, and thus requires using negative values).
If this is deemed intentional, MCPE-170535 should be reconsidered.
(To clarify, I'm not reporting this as a parity issue, but rather behavior contradicting a mojang statement.)
Commands are not in parity between engines. This is a feature request.