That only happens (generally speaking) with the sportsbooks where the fiat currency is BTC. You're f'ed if the currency goes against you, and that can go the other way as well to be devil's advocate.
If you use a site like 5dimes, fiat currency is USD for Americans. So if I send them the equivalent of $1000 in BTC today (let's say 5 BTC for example's sake), I get $1000 USD in my 5Dimes account, +/- flux over the course of ~1 hour, which should be minimal to nil. If in two months time I win $500 grinding out NFL plays and hit a nice streak, 5dimes sends me the BTC equivalent at that time of $1500, and I have ~1 hours worth of FX exposure while the BTC moves between 5dimes and my BTC wallets.
Now, Nitrogen Sports and sites like them use BTC as the fiat currency, meaning BTC is the currency you hold and bet. THAT is when you play with fire. In the previous example you deposited 5 BTC and bet in denominations of BTC. Say that you win 50% over time, you would return 7.5 BTC. If BTC loses 25% of its value in that time, you're taking a $375 loss against your $500 winnings, almost wiping out your winnings.
Basically, use books like 5Dimes/Sportsbook/Heritage/etc. that translate into USD upon deposit and settle you in BTC at the USD amount you withdraw at the time of withdraw.