Forcibly hitting someone with the crown of your helmet doesn't just apply as a rule to defenseless players. He's quoting Rule 12 article 7 (defenseless players specifically) when Rule 12 article 6 says:
Article 6: Unnecessary Roughness. There shall be no unnecessary roughness. This shall include, but will not be limited to.
(h) using any part of a player’s helmet (including the top/crown and forehead/”hairline” parts) or facemask to butt, spear, or ram an opponent violently or unnecessarily
To me this clearly defines his hit on Lacy.
He got away with the Starks hit because Starks' helmet lowered a bit when he braced for the impact. To me it was obvious Meriweather was trying the same thing he did to Lacy but I guess the NFL can't really try and judge somebody's perceived intent.