Online gambling sites = invite the public in to throw money at each other, then sit back and rake profits off the top.
With fantasy sports, it's unlikely that cheating mid-game could be hidden from plain sight (unlike poker). That would involve changing rosters or adding entries after games start. The other criticism of poker sites was that some players were entering tournaments multiple times with different IDs. Fantasy sports sites allow that, and even encourage it, so this is another "plain sight" factor - if you enter a contest only once, you should be fully aware that someone else might have bought a ton more lottery tickets than you did.
So, if there is a player with a ton of entries who is winning often, I see two scenarios:
1. That player is a member of the public who invests a lot of money and time, and therefore makes a profit by sheer volume of transactions. The same thing happens every day in a capitalist economy. It takes money to make money, and the cards are stacked in favor of the richest 1%. The other 99% are a longshot to change their fortune.
2. That player is "owned" by the house (the gambling website). If those entries are seeded by the house, they run the risk of having to pay out more than what they take in on a contest, ruining their rake.
It seems to me that scenario #1 is far more likely. Either way, the odds are against the casual gambler who just puts in a handful of entries a week.