Question: When a question is closed as being *off-topic* or *not a real question*, the question is automatically down-voted. This was instituted network-wide after some discussion [a couple of months ago](https://meta.stackexchange.com/q/84978/149432).

It seems like this behavior should extend to *not constructive*, which is arguably worse than an off-topic question, at least on Programmers.SE.

Are there any reasons why it wouldn't or shouldn't?

Answer: 
There is a difference of intent.

If a question is *off-topic*, that is akin to asking about tea on `coffee.stackexchange.com`. It's a blatant, obvious violation of the parameters of the site. Now, you could argue that users simply don't read, and I will agree with you, but ignorance of the law is not an excuse. But in this case, *it is egregiously off topic*. That is, even the most casual browsing of the front page would immediately let you know what is and is not on topic without thinking too hard about it.

When a question is *not constructive* the distinction is much subtler. It can take a bit of trial and error to "get it". I also believe that many unconstructive questions do NOT indicate the same level of obliviousness bordering on irresponsible failure to just *look at* the questions on the homepage and ask something similar. I believe it is possible for some people to honestly try to read the faq and still get it slightly wrong.

Therefore I don't support this.

(However I do think that egregious examples of unconstructive questions should regularly be downvoted and remember, downvotes on questions are now free... [Should downvotes on questions be "free"?](https://meta.stackexchange.com/questions/90324/should-downvotes-on-questions-be-free) )
I've never understood the difference between 'off-topic' and 'not constructive' - they're synonymous to me (i.e. a question that is deemed on-topic *is* constructive by nature of the requirements stated by the FAQ). 

So, no, I don't think there's any reason not to follow suit - 'not constructive' votes *should* incur down-votes.