Let me guess, you haven't read the terms and conditions when you signed up to sportsbet. Instead of waiting that your transactions hade been confirmed, you started immediately spending them by betting them. Later on the transaction is still unconfirmed and pending but all the bets you made with it are allready finished. Now what company would pay people their winnings when they still haven't even received the bets initial money yet? They should pay it out of their own pockets? That would be absurd. What if your send BTC never confirms because the fee of it is so low no miner will touch it unless the blockchain transactions is so low at the moment that they will, with more and more microstransactions going on in BTC the more unconfirmed transactions are piling up. Next time at least wait untill your transaction has been confirmed before actually using it. See it like this: Your job transacted your paycheck to the bank to make it out to you, the bank usually takes 3 days doing this untill you see it on your bank account, since we don't see the background of it we think it instantly shows up in our bank account. Now are you able to spend this money before it has actually reached your bank account? No. Yet you did on sportsbet. If you look at your situation from an objective point of view, it appears more likely that spending unconfirmed money falls into the scam category.
Also the whole situation could have been easily avoided if sportsbet would only allow you to make a bet only untill after the money cleared, but since todays people have zero patience mentality they allowed this, again see their terms and conditions, which obviously nobody reads.

So basically what happened was you won both of your bets, but what sportsbet.io are offering is simply a full refund on your deposits + a 50mBTC compensation compared to an otherwise much bigger win and profit?

What evidence did they even have to accuse you of double spending in the first place?

Providing some screenshots as proof would be nice as well.

sportsbet.io does not play fair by all accounts. I doubt if you will ever get your winnings. But one thing is for sure, had you lost 8the bet, they would have kept your deposit after confirmations. Good website to play on, nice UI and all that, but their underhand practices leaves a lot to be desired.