This was the best possible outcome you could have had in this situation. I'm shocked you got paid it all. If you lost the bet, they could have taken the bet and your wager. If your bet won like it did, I would have assumed they cancelled the bet, refunded your money and told you off for betting bad lines. I did something similar to this several years ago betting a baseball game live on bovada and they cancelled it when it won. I didn't know any better at the time.
For what it's worth(absolutely nothing), I think its wrong that this is how it is. They made a mistake and should have to deal with it. But I think all online books would handle the situation in the manner which I laid out. You would be better informing them when they post bad lines instead. I informed bet any sports a few years ago about a bad golf line and they gave me a $25 free play. Hopefully this helps. BOL