Merit sources are not limited to a particular board, but it is necessary that most of their allocations goes into the board which they were assigned for as merit source, what's more important is that merits are spread across quality posts.
I don't remember any case merit source got punished for mainly sending merit outside board they mentioned on their application. So IMO word "expected" is more appropiate than "necessary".
And why bitcoin discussion and economic boards lack enough merit distribution, is because merit sources consider those sections are full of spam so finding good ones might be rare and even most of them ignore those board.
Not only that, Bitcoin Discussion also contain some thread that is probably more suitable on other board. Few example from page 1,
bitcoin vs real estate probably better on Economics
They might begin bitcoin mining death spiral fud again after this news probably better on Mining
Can peace news really make Bitcoin bounce back? probably better on Speculation
User such as
CTO114 frequently create generic thread, mostly on this board. His profile page shows he create 161 thread and reply, but if you click "Show the last topics started by this person.", you would find out he create 134 thread. I don't bother report any of those, so i expect most user have same attitude.