This is a good idea; one way to look at taxes are market signals.
By subsidizing healthier foods whilst at the same time taxing unhealthy foods; we signal to the market what we want to happen.
In my opinion; this is far better than either outright bans; or the current free-for-all system.
One could easily paint it as social manipulation; but the government engages in this kind of manipulation all the time anyway.
That is to help with the seamen sample collection, obviously.