PGL, you do pose an interesting point here, when talking hypothetical situations. That include the notion of "completely disregarding the entire situation", as we don't know what all actually happened in that car, so what you are saying is the "entire situation" is still hypothetical. Just as you leave out the possibility he did more than restrain or attempt to restrain her, considering she was claiming he hit her too (don't know if he did or didn't, but it is not beyond the possibility).
In the hypothetical situation you described above, if the events were he's just driving down the road, his GF is in a drunken rage, and is that person, sure I'd say it's reasonable to have to attempt to physically restrain her, physically holder her back, which could unfortunately result in some physical injury. The picture you are painting in this hypothetical is essentially self defense.
Most others are basically giving their own hypothetical that what occurred was not that. I have seen few and far between where anyone is out right saying Manziel is just some women beater, only that they leave open the door for the possibility that he could be, that what happened in that car wasn't as just self defense, none of which is unreasonable. The hypothetical you paint is not to far off from the hypotheticals people painted for the Ray Rice incident, until that video came out. We just don't now what happened in that car, so there are going to be a lot of "what if" discussions.
Going all BS on this, no one is saying a one way street exists where it's a "never lay hands on a woman" because they are some fragile creating. Just as I don't think anyone is saying its a free game for it to be okay for women to lay hands on a man. Or okay for men to hit men, or women to hit women. For me, it's as simple as this, there is never a valid reason for physical violence to begin with (that does not rule out self defense, as if no one started the physical violence there would be nothing to defend against). Through the history of our world violence has never solved actually solved anything. Violence just results in more violence. There is no situation where actually rationally discussion out an issue that it cannot be solved. Reality is that's a pipe dream and an ideal world, which I don't see existing anytime in the near or distance future.
It is unfortunately a "normal" situation in our world for there to be physical abuse in a relationship between people, be it man abusing woman, woman abusing man, man abusing man, woman abusing woman, etc.
As to it being a normal thing in relationships you get in, well, all I could really say to that is you might want to re-evaluate your choice in woman, and what you want in a relationship (i.e. wanting drama).