It's cheating, but it's a silly rule that everyone has ignored until the Patriots do it. There shouldn't even be an inflation rule—footballs obviously have to be inflated to a minimum level to be of any use, but otherwise the league should just let teams inflate to a level they're comfortable with. There's no difference between softening a football and wearing supergrip gloves. This is all so silly.
And if the league is going to punish the Pats, they should also punish the Vikings and Panthers for warming up balls during a cold game in November, and they should punish the Bucs because a decade ago Brad Johnson "doctored" the Super Bowl footballs, and they should punish the Packers because Aaron Rodgers admitted to trying to sneak overinflated balls into games, and they should punish Eli Manning for filing the laces on his footballs, and they should punish ...
You see where I'm going with this?
But it's the Patriots, so the league will make an example of them. And then no one will think badly of the NFL anymore.