The IRS attitude toward "evil" may be part of it, but GREED is even more of their motivation, IMO.
Every level of government likes more taxes, and it's easier to take a piece of gambling winnings than a piece of a paycheck.

It's like the explanation in one of the Godfather movies, set in Cuba. One of the corrupt locals remarks:
"You and your friends should show me some respect. You should let me wet my beak a little."

And so the IRS levies their tax.