That lock-happy mod is the one of many fighting to keep these questions somewhere for people like you who push these silly conspiracy theories.
Stop it. It's annoying.
2. The mod took action but all mods took chances putting historical locks on question. There were many questions to clean up. So the mod didn't say squat, it was action by the mod which displayed a message by the system. Get it right.
3. Placing a question with a historical lock instead of closing and deleting is fighting to keep it in the system as much that is hard for you to believe seeing your extreme bias to mods of which you don't really what they do and why goes in the background. You really need to understand how this works instead of these claims. Mods see them, Stack Overflow users see them and they know it's false.
So straight to it,
_Why is it not a valid Stack Overflow question?_
Because it's not _definitively_ answerable. Keyword: definitively.
That's all there is to it. A 1-to-1 relationship with a question and acceptable (to the author) answer. Yes it's a programming question, it's a great question, it's an interesting question, it's a popular question _but_ there is no way to make an answer for it that can be acceptable without clumping it into one answer for the author. Everyone can have their opinion for the pros/cons. Stack Overflow is for definite Qs&As not discussions.
Once again if this wasn't realized this question and those like it aren't going anywhere. It's archived. Please re-read the notice and stop calling out mods, they do a lot (and by a lot, I mean enough that they should be paid for doing it) of moderation work for free. They don't need unfounded claims.
See more at http://meta.stackoverflow.com/questions/126420/what-to-do-wi... and http://meta.stackoverflow.com/questions/126587/what-is-a-his...
On your #2, it seems you're saying the mod has to call it an off-topic question even if he/she is locking it for another reason. If so, then it wouldn't be the mods' fault, it'd be the SO developers'. It's still a path to ruination.
I disagree that a valid on-topic question for SO should be limited to definitely answerable questions (in this case, where a pro or con can be proven). If that's the case they should make that clear in their FAQ; I don't think they do that. While that can be their unwritten rule if they want, it makes SO much less valuable for programming questions for me, because not all programming questions I find valuable are definitely answerable, like the one I referred to. 400+ people (on net) found that question to be valuable. I don't think a question asking about pros/cons for a programming design decision is a discussion question. A discussion question would be more like "Would you store images in the database, or just links to files?" Even someone who prefers the former could potentially list out pros/cons for both choices.