aOK - you and I usually agree and I'm definitely a rule follower. But sometimes in these types of situations I think the 'rules' do have to be ignored. Sadly, there are plenty of criminals running around because someone forgot to read them their rights or evidence was improperly gathered or, I don't know, list a reason. It happens all the time. Technicalities shouldn't aide and abet.
In this particular case, based only on info provided here, I think the correct actions were taken. Regardless of RRO, by-laws or codes of conduct, when grown ups behave so terribly and children are adversely affected immediate action needs to be taken.
If this does prompt their PTO to include removal clauses and the school to adopt a Volunteer Code of Conduct then that's terrific. But until then, I don't believe the absence of a rule means no action can be taken.
Mostly, as others have mentioned, I think the originator of this post, wlovelace, needs to step back for awhile. Pushing herself upon a group that clearly would like her to take a break doesn't help at all.
Regarding the original question,
Is there some sort of by-law that I broke?
That depends on your own group's bylaws. But, again, does it matter?