With legal sports betting you must pay for your bet up front with real money. You can’t bet it if you don’t have it. With legal, everything is regulated by state commissions . There is a certain transparency that allows very little room for fraud and fixes. The illegal side is different. Most bookies operate by extending credit and taking bets over the phone or through third parties.
A gambler may end up owing far more than they are capable of paying. There is a culture of squeezing the debtor. If you can’t pay the cash, then maybe they’ll take the car, diamond ring or plasma TV. Perhaps, if an athlete, coach or manager is in trouble with illegal gambling, they may be tempted to throw a game to make good with the bookies. Not everyone lives near a legal sportsbook where the betting is legal and regulated. Not everyone is comfortable sports gambling over the internet which is somewhat legal and somewhat regulated.
I say somewhat because the internet is a hard place to enforce jurisdiction and not all the overseas countries where the sportsbooks are based regulate much. The official rule in this country is that it is illegal to place a bet over a phone line.