Yes, you're allowed to do that. Going by that logic, should we allow all kinds of spam because it is (potentially) less dangerous than downloadable links? Hopefully you start realizing how faulty this argument is.
No Sir. Please just explain me how can referral link as part of OP be spammy and sending it via PM is not spammy? (though it increases the work unnecessarily for the person who created that thread to get some referrals and pay them)