One of my very first posts to this forum (well, the old forum) was about someone who wanted to homebrew a fuel level sensor for a gas tank in a car; I remember strongly urging them not to play around with such a thing, because of the possibilities of a gas-vapor ignition, possible insurance issues, etc. They chewed me out for being overly cautious. I just didn't want them to become a burn victim.
I'm pretty much with you, GM, on this - if it is something that I think the poster has no sense doing until they have much more knowledge, then I would say the same thing you did. That thread was not a beginner's project. Unless the poster wasn't telling us something (like, maybe they were a beginner with the Arduino, but had 10 years of PIC interfacing and coding experience, for instance) - they shouldn't be attempting a cruise control system (at least not for a full-size automobile).
I'm always pushing for safety systems and failsafes for people building larger-scale robotics projects, too. I'm sure some of them think I am being over-cautious. I know I'm not - that is the kind of system you design and build in first, not later, because like all security systems it has to be put in at the lowest level, before the rest of the code - otherwise it becomes a band-aid solution that can easily fail (whether it is a small embedded system, or a large many-100K line online transactional system). Too often I have seen the latter done, though, rather than the former - because the former was either boring, or (more often than not) it would take up a lot of time and cost a lot of money for the project (only to find later, after something got hacked or whatnot, that it was needed - and then it took 4x the amount of time to implement the band-aid solution after the fact, and it never works right properly then anyhow because all the holes couldn't be found).
Not quite the same (certainly not on the level of a "moral question") - but I often waffle on whether I should give help to someone who (it is obvious with their question, generally) hasn't even taken the time to type a few keywords into google. Someone the other day had a question about how to get the Arduino to talk to a Vex ultrasonic sensor. I first wrote out a reply with links, asking why they didn't bother going to google (as I use three keywords, and the first link that came up went to a Vex forum that explained how to do it) - then I started to re-write the answer without giving the links - then I just gave up and didn't post anything at all?
Should I have posted something? I don't know; how is it that I can simply type these simple three words into google (all three words were easy to figure out) and find the answer on the first link - but this poster had problems doing that? Did these people, back in the day, just walk around libraries going "hurr durr" and harassing the librarian on how to use the card catalog? I suspect they did. Now, granted, card catalogs (how I miss them sometimes) weren't super intuitive, but they weren't nightmares, either. They certainly weren't google, though. Google is far and above easier. Type your keywords in, get a response! Why is this so darn difficult for people to do?
/end rant