When one of my flags is accepted, the helpful flags count is incremented, but I do not get a notification.
Is this intentional? If so, why?
Yes it is intentional. If you are only flagging a few things then the notifications would be manageable. But if you find and flag lots of things then the notifications would quickly become annoying.
You should also think about it from a UX perspective - acceptance of one of your flags is not something that requires an immediate reponse from you, nor is your life changed drastically if you don't find out about it immediately. You might get a warm fuzzy feeling from it, but if that is what you are after then wouldn't it be better if you just waited till the end of the day and checked your stats then?
You should also think about the amount of extra (system) work that goes on for this - most of the time you are not the only person flagging something, so you wouldn't be the only person that needs to be notified - that equates to a lot of unnecessary traffic and webserver/database CPU time for a feature that accomplishes nothing.
You don't receive notification for a flag being accepted. Period.
There were occasions that I used up all 30+ flags available per day, and I don't want the notification to be flooded with such useless information.