From what I have read Charlie Whiting has declared it legal, so it is legal. Watch the race now for everyone else to develop a similar system.
Can't argue with that.
Various reasons for its acceptance have been given;
Charlie doesn't want to stifle all innovation, and its fairly simple to copy, so won't favour the front-running teams.
Ross Brawn has mused that a lot of the complaints from other teams, whilst they quickly develop their own versions, draws attention away from their questionable exhaust systems.
As far as I'm concerned, it's a storm in a teacup. The "primary function" of the DRS button is to reduce the drag of the rear wing in order to allow a higher speed. The "secondary function" of the DRS button on the Mercedes is to reduce the drag of the rear wing in order to allow a higher top speed.
Given that a driver needs to carefully select his gear ratios, for the best compromise between running with and without DRS activated, I don't think the tweak, available only when DRS is activated, is a major advantage; remember, Mercedes were quick on the straights anyway.
I can't see that the small advantage offered to Mercedes, only in overtaking in the DRS zone remember, would be of much greater value than a driver that is more skilled in overtaking!
Given the choice between Kobayashi in the driving seat, or the DRS F-duct, I'm not sure which I'd prefer!