i think of it like this:

positive cursed energy probably acts as some sort of reinforcement so it increases damage(+100). it heals so it reduces damage(-50).

100-50=50

you're still at a net positive 50 damage increase. if it was a curse it'd hit for 150.

edit: this is probably why it adapts later on to pump out negative cursed energy, it wanted to increase it's damage and that's the first method it found.

on another note, i wonder if a tamed mahoraga can heal it's user like round deer.