They are losing money giving it for sure.
Bet they're not - actually it's very clever...
My PTBB at e200 games on B2B with a win-rate of 6.4PTBB/100 shows $2700 won and $1500 rake paid. So if I'd stopped after exactly $2000 winnings my rake would have been $1110 - they still show a $110 profit on my play.
You are going to need to sustain a PTBB/100 of over 7 on the $200 tables to start costing them money - what % of the player base who are drawn to the site by that bonus are actually going to do that? Very few I would guess..maybe at much higher stakes the rake is proportionately lower but then so is sustaining a high PTBB/100.
Of course it's true that they are only a skin and so will not keep the whole rake for themselves but the principle stands.
Very innovative bonus - like it