A bridge collapses and people die. In the resulting lawsuit a licensed engineer says, with many pages of reports, that everything was done correctly and the collapse was "an act of god". But an amateur who only knows a little about civil engineering points out one single thing they did incorrectly; he doesn't know everything about engineering, but he does know this one thing they did was wrong. The government then charges the amateur with a crime for knowing a little and speaking about what he knew.
That leaves the risk of a board ousting (or implicitly promising to oust) people based their expert testimony in court. Which would fall back onto the board's rules about revoking licenses. Lawmakers would likely get involved if any funny business happened; revoking isn't an obscure bylaw, but one of the two major purposes of the board (giving licenses being the other). I'll admit this hypothetical gets too complicated and delves too deeply into politics for me to guess likely.
Do you know of any historic examples besides police unions? I'll admit they match your worry, but the police benefit from massive public support that borders on mythology. So it's hard for politicians to do much about those problems.
That said, I’m not sure if being licensed means anything more than ‘I paid my dues to the state board’.
Take a test
Study under an actual PE for a period of time
Take another test
Pay some money
Do some work
I studied Engineering in school, but I didn't get a PE because taking the test was too much effort and I wasn't going to work with any PEs, so I assume that actual PEs know stuff that I don't know.
The right and duty of every citizen, what's your point? The oath obligates you to tell the truth, people testify about fraud and murder without being experts.
"Is giving a speech about how to treat a broken bone considered ‘practicing medicine’?"
Giving the speech in a court, on the witness stand, under oath, claiming to be an expert in treating broken bones?
How about an example from engineering? A bridge collapses and fifty or sixty people die. In the resulting lawsuit, Bob, a nationally recognized bridge engineer, testifies, with many pages of reports, that the design of the bridge was substantially bad. Ted, who has a degree in engineering and worked for 20 years as an engineer (designing coffee makers, say) testifies that according to his calculations, of which he presents a plethora, the bridge design was entirely adequate and the failure was an "act of god".