I think it's not allowed because of a few reasons:

1. If you allow ++, you should also allow -- - which means that you can mob other monks
2. It could easily be misused as sympathy "karma"
3. You'd have to invent rules how often you're allowed to ++ or -- another monk, and you'd have to implement it.

I think a sane workaround is to simply pick a node from the monk you want to ++, and ++ the node instead. That's not perfect, but less dangerous in terms of abuse.