Yes the bonuses are for BitEdge referrals only so you have to sign up through a BitEdge referral link to get them. Some forums don't like people posting their referral links so I just posted the links to the bonus section of our site instead.
I have accounts in all the sites that you mentioned except for Bitstarz(Bad reputation thanks to the scam accusations against them) which is why none of these bonuses are particularly useful for me and this is probably the case for majority of the gamblers in this forum.
Some gamblers who don't have accounts in those sites could sign up through your links though. However, you are bypassing the referral rule in this forum in a smart way which could attract negative criticism.