Because children, especially toddlers like two year olds, rarely comply with a requirement to sit (just the way this OP's charge did not sit when requested to while standing on the chair). This requires forcing them, then they defend themselves, which makes the adult angry, which lead the adult to require them to sit in any way they can....then the shoving happens.
Not saying anyone here does that or did that. Just a conclusion from working with typical and non-typical kids in daycare and respite care settings for a decade or so. It informed my own choices with my own kids.
I would have absolutely taken the child out of the restaurant for a bit, but probably would have picked him up instead of walking with his hand in mine.
I'm not against physical separation and firm addressing of aggression issues, I'm just not particularly for traditional, "Supernanny-type" structured time outs.