Just a quick PSA, courtesy of our friends over at r/churning:
Online sports betting transactions with sites like FanDuel are typically considered a cash advance with most (all?) credit card issuers, such as Chase, Amex, Barclay, etc. This is especially relevant now that online sports betting is becoming legal in more states, having recently become legal in New York.
Apparently, fantasy leagues are typically not considered a cash advance, per this Yahoo article, but betting on a particular game is considered a cash advance.
Update: Also read through the comments as it appears there are some sites and some cards and some types of bets that do count as an ordinary purchase.
Source: doctorofcredit.com