Even if there is no house edge like in sports betting
There is always a house edge even in sports betting. If there was no edge they would be doing the service for free, the odds they quote include their fee and they gain a percentage of bets placed by doing so. Even when people win they get slightly less then the losers on the same bet lost, so the house or sports book makes the difference between the two and cannot lose overall like a bettor does.
Thats presuming the book is balanced, I guess sports bets also get alot more bets one side then the other. Its upto them how they handle that risk or maybe just close the book if its going too far one way but usually they keep making the odds worse I think. Anyhow every betting operation is taking fees.
Same deal happens with foreign currency exchange, especially when they insist its a free service. They make a ton of money on the two rates not matching evenly. Thats normal competitive capitalism, its a skill to spot the best odds I guess