It isn't. But the NFL and DirectTv both market their own streaming products as well as the Sunday ticket itself so they would be against any streaming that wasn't putting money in their pockets. Slingbox is perfectly legal and has things in place to keep it legal so talking about it here should not ruffle any ones feathers. But the rest of the streaming is considered taboo here, either out of fear of NFL action, not wanting to deny the NFL of it's profits, or just feeling that it is immoral/illegal.
And if I may ask, if you use slingbox are you able to get HD content streamed anywhere outside of your home? If so, what speed is your outbound internet? And I read that the box has no HDMI inputs which seems insane to me, is that true? Thanks.