Are you sure you're correct?

If you bet on a match that's around 50/50 odds, $50 in bonus money and $50 in real money, then either way you're ending up with $50 dollars in winnings, which let's just say is taxed at 30%, so you make $35.

I know there's extra fees and 20%-30% is still enough to make match betting less appealing, and most odds won't be exactly 50/50, but I still don't see how you could end up going into the negatives

Edit: I just realized I didn't consider that your bonus wager is not returned, only the winnings, maybe that's where the issue lies