That's true but if you punish Dak for spitting on the ground, you also have to further punish Carter for provoking that response verbally and by stepping to the Cowboys players. None of it was physical except for what Carter did that merited its own punishment.
And there is no way people aren't charged with assault just for spitting. They may not be convicted or receive much of a sentence, but of course it happens. (Also happens if you do it to a cop, of course.)
Of course the Irving Police Department, in tweeting about this to troll Carter, also noted that spitting in a public place can be disorderly conduct.
In any case think bringing Dak into it just attempts to excuse or obfuscate what Carter did. He's been punished, he's taken responsbiility, and I suspect there is still internal discipline, though it's hard to see them letting him miss a series (as with the last time) in such a big next game.