I'm sure the rule wasn't there few days ago. When I signed up, I read the terms carefully and the only rule was to have FJ sig in first line.
Looks like the rule was added in recent times, since the OP seems to be edited.
I'm pretty sure it was always there. Check the post #47:
@FortuneJack
I asked you in pm, but others will probably want to know too - are we allowed to put referral links in the signatures?
No, as it is mentioned in the OP
"no other advertisements allowed" including our referral system
but you are more than welcome to participate in our referral program and earn 5% net profit from the players you invite. You can do this by sending referral links to your friends or PM people in this forum, or any other way you find it possible.
Feel free to ask us any questions you might have about our referral program or website.
He was referring to this rule on 12th Sep.