The thing is, there's a demand for it - ask the next WG you see how often punters ask for BB at beginning of/halfway through punt.
If your a WG, perhaps drugy, or giving half earnings to Sergei, or having money problems, or crackers, or have done it a few times without getting STI, or whatever. Then you have some guys offer you an extra £50 or whatever to cum inside you, it must be tempting to say yes and then your one step away from advertising it!