Of course we all know that when a personal pronoun is the object of a verb or a preposition, it must be an objective pronoun. My question here is about the choice of a nominative pronoun in the slogan of the solidarity movement for gender equality "He for She". It sounds ungrammatical to me and I wonder why it was preferred. Can anyone explain?
This is not a sentence where applying grammatical rules is obligatory, but merely a title for a campaign aiming to raise awareness. Hence it is only logical to pick words that will make the title catchier and somewhat easier to pronounce.
The "he" and "she" are being mentioned, rather than used. The quotation is implicit: "'He' for 'She'". Quoted forms are not subject to grammatical rules. If they were, for instance, you might have They said "Harry likes turtles." getting a passive form Harry was said "likes turtles."
I generally see it spelled HeForShe, and if the "She" is just part of a word, that is another reason it can't be changed to "her", since parts of words are also not subject to grammatical rules.