Marijuana legalization arguments were my first introduction to motivated reasoning. I was pretty inclined to agree that locking up non-violent drug offenders was a net-harm to society. But, the pro-legalization folks would argue patently crazy things: it cures cancer, the smoke isn't bad for you at all, there are no downsides! etc. It seemed obvious to me that you could make a more realistic argument and just stick t…
This could be a person making a bad argument, or it could be that the individual is the opposition trying to poison the well. Cf COINTELPRO. Largely any movement has people with insane takes, and it's impossible to tell the difference between good and bad faith actors.
That, and sometimes people just aren't trying to be persuasive at all. It's extremely rare to actually see someone persuaded about anything political without enormous amount of effort, or more realistically a change in material interests.