For the same reasons being a coder isn't necessary to criticize code. Anybody can do it, but chances are your opinions are a lot less informed than you realize.
Likewise, a non-designer offering opinions on design philosophy or alternate styles is probably pretty useless. A non-designer saying the thing is awkward/ugly/confusing/whatever is informative though.
This is not the same for UX (or restaurants, films or cars from my other example) as these are things that everyone interacts with and understands to some extent despite not doing so in a professional capacity.
A practitioner who ignores feedback and says "who's the UX professional here?" is not a very good one.
A non designer is not going to be good at identifying solutions to problems because that does require knowledge about how the sausage is made, but they should certainly be listened to when identifying problems. Not doing so would be madness.
(Not to mention a lot of the commenters on here that you're decrying as "non designers" probably do have experience with design, considering the userbase of HN)
This disease infects an awful lot of "modern" web sites.