Big thing here is...the guy who left the note was making a valid point. It's his right to share his opinions in a legal, safe, non-threatening manner, just as it's your right to take the paper he wrote the note on and wipe your arse with it without giving it a second thought. No harm done on either side of the interaction.
Had the guy waited by the vehicle and then told the fisherman that they were stressing the trout...that's fine too.
Had he waited, then laid into them with a tirade and threats and insults? Not okay.
All in all, there was absolutely no harm done, and anyone truly offended/upset by it is probably taking it a little too seriously.
Do I try to avoid fishing 'marginal' waters when the water temps creep up? Sure. Will I still do it if that's what I really want to do? Probably.
I was out trout fishing Saturday in high 80s, low 90s, (with a 4wt! the horror!). Caught a 17-18" trout, photo-ed and released him, no problems at all. If someone says you gotta play a big fish for 10 minutes on light tackle, it's because they don't know how to do it properly. From the strike to the release of that trout, on a 4wt, with photos...maaaaaaybe 3 minutes.
Had I had a similar not under my wiper when I hiked out, I'd have laughed, crumpled it up, and tossed it in the little bag I keep for trash in my car. Had there been some dUUd up there trying to teach me about how I was killing the trout, I'd have heard him out and told him my view. Had that dUUd spazzed on me, I'd have probably told him where he could stick it...then showed him the fish my friend kept. :P
It's all about the delivery, and all things considered, this guy's was sensible.