Like you point out, people could get potentially be fired for walking out. At that point, those employees have demonstrated that having this clause changed is worth potentially not having this job. If that’s the case, why didn’t they negotiate on that clause before they were hired? Same things at stake, same gambit, just more direct.
Furthermore, now that these employees are trying to renegotiate after having worked with such terms in place for a time, what prompted this change? Were they once accepting of such terms but changes their mind? Or did they not object when that clause was only in their own contract, but now that they see it in other peoples contracts they are inspired to make some company-wide change? The motivations are far from clear.
The other part of the above comment left unaddressed is the implicit demonstration of priorities seems strange. The games industry has plenty of turnover, burnout, and rough conditions. So when some people choose to work in the games industry, presumably out of passion, their opportunity cost is substantial. Meanwhile, Riot Games has had a history of bad/abusive behavior within the company. So what kind of person does it take to go work at Riot but later decide “this is the issue where I’m taking a stand.” If they cared that much about working some place they like, and Riot already has this history, it seems strange to later be indignant about their workplace.