I'm also a little surprised that no one has sent them a cease and desist letter yet. Pretty much every web site in existence forbids this sort of thing. For example, one broker
says this in their T&C:
Smaller brokers probably don't care, because it gives them exposure. But the larger players could potentially see this as just a way to generate visibility for the competition that spends less on advertising.
Edited to add: For example, when I read this post, I see that DISBoards automagically inserts a referral link to DVCRM, and I'm pretty sure that DVCRM pays for it in some way.