I think this restriction does not solve the problem of shitposting, 1 Merit not enough.
People will just buy it from the high-rank sellers for their army of junior member. I personally saw today a guy bought 60 merits for his farm in the telegram chat.
If we want to stop an army of Junior member bots, we need to add more restriction.
As I see the main problem is bounty campaign with encouraging people to make more post or bump ANN tread.

There a few solutions which I see:
1) Don't take Jr. Member in signature bounty only member+.
2) Too many messages with social reports and  "joined"? Encourage all bounty managers to use Google forms for weekly reports.
 
To implement this we need the strict rule for bounty manager to not encourage shitposting.