Speculation on why Ranma allows people to abuse him

[claymade]

Slapstick. Ranma "allows" people to hit him for the same reason Happosai "allows" himself to be pummeled by non-martial-artists, or Ryouga "allows" himself to be spatula-ed by Ukyo--because it's a slapstick comedy, and that means that, in comedic situations, people will take out their aggravation on others by means of ridiculously exaggerated physical violence, irrespective of their target's actual skill level.

Doing that's is perfectly fine if that's what you prefer, but I think consummately labelling the series as 'just slapstick' severely limits potential reader investment in the characters and also character analysis and study by those same fans. After all, isn't that why we come to these forums? ^^

But in order to analyze and study, you have to understand things in context. You can still interpret and empathize characters within the framework of the slapstick world-rules, but unless you're interpreting it in that light, you're going to get a horribly skewed view of the characters.
The Ranmaverse is a place where having holes blasted in your wall by the Ramen delivery girl is a minor annoyance. It's a world where you can get your neck snapped at a 90-degree angle and then just walk it off. It's a world where an entire army of students will attack a single student--who doesn't even want to fight them--in the schoolyard every single day in plain view, and the school officials don't pay any apparent mind to it.
What do you think of when you see Ranma do things like drop Kuno headfirst out of a tree in a fit of aggravation when Kuno insults him? Or drive Kuno's face into a table just as a way of saying "hi"? We don't consider him a monster, because we know that Kuno isn't actually going to break his neck or get permanently injured by such comedic antics.
Unless you make the fundamental slapstick leap that random acts of comedic violence just fundamentally aren't as big a deal in Ranma's world as they are in this one, the whole thing changes from a romantic comedy into a sort of ridiculous dystopian nightmare.
Personally, I think Takahashi does a fine job of distinguishing the moments where she's showing us the world through the goggles of slapstick physics from the more serious ones where it's less so.

Wrong, this is ignoring many circumstances. First of all, Ryoga, knowing that Akane is engaged to Ranma, and Akane, knowing she is engaged to Ranma, should not be dating. Ranma, however, has conflicting engagments, not of his own fault (Kiss of Marrige was given _in Japan_ protecting Akane, Ranma had no responsibility to know what his actions would have caused until after the fact

Except that nobody except the Amazons regards their bizzare little law as in any way binding. Ranma says he's "not even thinking about marrying Shampoo" and considers the Amazons' schpiel to be "looney talk."
Likewise to the Tendos' engagement--even before all those conflicting engagements became known, he'd already indicated to Akane that he had no intention of actually ever marrying her. So if he doesn't, then why should he be allowed to chain her to his orbit while he goes out and dates anyone he wants to?
I see no indication whatsoever that his antics with Ryouga were in any way related to some selfless devotion to honorable principles of engagement. As far as I can see it was simple jealousy, and over a girl he hadn't even had the decency to admit to actually loving, even if he was going to covertly sabotage her chances with other guys in utterly cruel ways.

Ranma's actions against Ryoga were completely justified, he was protecting the honor of both Akane and Akari, whom Ryoga was taking advantage of...

Um, correct me if I'm wrong, but isn't the Yoiko story arc in volume 11? And doesn't Akari show up in volume 30?

[lwf58]

Um, correct me if I'm wrong, but isn't the Yoiko story arc in volume 11? And doesn't Akari show up in volume 30?

There are two arcs in which Ranma-in-drag tricks Ryouga in his own house.
In the first, Akane goes to see Shirokuro's puppies, and that's when Ranma fakes that he's Ryouga's unknown younger sister. In that one, it's sheer jealousy that motivates Ranma.
Later, in the story arc where Ranma and Akane are helping Ryouga and Akari get together as a couple, Ranma pretends to be the Hibiki's housekeeper. That time, it's a little less jealousy and a little more a sense of indignation that Ryouga is trying to kiss up to both girls at once that causes it.
Both times, Ryouga -- who is shown in throughout the series as being unable to recognize female Ranma in even the flimsiest of disguises -- takes it for granted that such a person could exist because of how seldom he sees the rest of his family.