It's not banned because of it's ability to mask steroids, even though it apparantely does have that effect. It's banned because ALL diuretics are banned by the NFL, not just that one. Because players have weight clauses (body fat %) and expectations both contractually and verbally from their teams, some are tempted to take water pills, diuretics, or anything else they can when they get close to their limits. Same reason as diuretics are illegal in all the boxing commissions as well.
However I do have one thing to point out about it. No player in their right mind would use a diuretic to mask steroid use. Reason being in the NFL there is no warning, you go straight to a 4 game suspension for getting caught. It wouldn't be worth the risk of the diuretic not working for a test, and 2nd, the players don't know when they're being tested. YOu wouldn't live too long being on diuretics all day every day... your body would dehydrate and I don't think a football could pull that off for very long..
The drug in question can be bought at Walgreens for $12... nothing special about it.,