It's acceptable only because the authorities know the masses will accept it. And what could the masses do otherwise, gather signatures? So yes, it is how it is, until the masses somehow decide it's not - and then will get gassed again. Because the masses wishing for a change are rarely in a majority and authorities usually listen to majorities.
Well, by definition yes. The job of the masses is to resist by any means available. If it starts using terrorist tactics it stops being the masses. Of course besides gathering signatures there's many other peaceful adjacent things that can be done (leftist types love the meme of a general strike, but of course civil disobedience, road blockades, and so on are all on the table), and somewhere between guerilla warfare and spray painting stencils in the night there's a fuzzy ethical line. On one side it's Taliban style "we will wait until they leave" and on the other side it's again Taliban style, but the bad things.
Mostly the masses ought to be proactive, educate itself, and so on to avoid signing conventions that have these exceptions.
I think this discussion is missing a significant issue: why would a democratic government, of the people, by the people, and for the people, use tear gas? Why do British Bobbies carry batons?
The simplest answer, which is the basis of all police powers of the state: to prevent crimes against persons and crimes against property. Riotous mobs get people hurt, often killed. The businesses proximate to a protest, often small businesses, often under-insured due to cost constraints, are very likely to be severely affected by protests that turn violent. Major corporate storefronts can absorb the cost of damages, but the same damages to a mom-and-pop grocery could be the end of the business, no matter how much they sympathize with the community that is outraged. And let me reiterate what I started with: riotous mobs get people hurt, often killed.
Tear gas vs the baton is a lesser version of the observation by James B. Conant, president of Harvard University, in his autobiography: "To me the development of new and more effective gases seemed no more immoral than the manufacture of explosives and guns. . . I did not see in 1917, and do not see in 1968, why tearing a man's guts out by a high-explosive shell is to be preferred to maiming him by attacking his lungs or skin."
