The fact that bookmakers leave such a loophole is either deliberate fraud or sheer stupidity on the part of those who build the bookmaker’s lines—followed by shifting the responsibility onto the players.
You’ve raised a really important point, but I fear this thread will now attract a crowd of not-so-smart people trying to argue that “those are the rules” and “you just have to play by them.”
The same with the book can steal your withdrawals and deposits if they think you multi-accounted. I have no idea why people succumb to illegal sports book rules.
Let me hope this wall of texts is not pasted from somewhere.
If you want to do arbitrage betting or gambling and you are not cheating, use two or different gambling sites for it. If you are using the same gambling site for it, the gambling site will consider you as a cheater.
That's in-house arbitrage and that should be banned. Otherwise the book is just guessing and penalizing you for betting a good line.