The logic behind denying a user the ability to self-delete a poorly received question that has an upvoted answer is because it's disrespectful to the answerer to have their work zapped. The user who asked the question can not even disassociate himself with said question without writing an explicit request via email and invoking their rights under the CC-BY-SA 3(a)(1). Why then is it ok to have a moderator delete the question, and with it the well-received answer? Isn't that also disrespectful to the answerer that invested his time and energy into it?
It's generally assumed that users self-deleting a question with a good answer isn't good. Having a good answer means that there's some good content attached to the question, and the user self-deleting the question would be removing useful content from the site, so in these cases self-deletion is not allowed.
On the other hand, moderators and users with 10k reputation are entrusted with the ability to vote to delete these questions. Some questions are just... not good, and actively harm the site to have around, even if they do have a good answer/good answers. For example, if someone asked about washing machines on Stack Overflow, we'd want to remove it from the site, regardless of whether it has a good answer.
However, users very frequently want to self-delete their questions after they've been answered for bad reasons, and allowing them to instantly and singlehandedly do this would cause quite a few problems. Even allowing users to self-delete their answered questions when the answer hasn't been upvoted yet causes problems (such as this), and I think it would be a lot worse if they could delete questions with good answers.
TL;DR 10kers voting to delete answered questions tends to causes more good than harm, but allowing authors to delete any of their own answered questions would likely cause more harm than good.
Because the buck stops somewhere.
In lots of cases especially on meta - someone actually has to make the hard decisions, considering intent the harm/good that deleting a post does and so on.
Some time back, a hot question on Server Fault ended up being a poor attempt at trolling and was deleted. I lost a chunk of rep on it, and when someone expressed it seemed unfair - well, I personally was fine with it, considering everything.
So in this case someone reviewed the question and the tone of it, felt it did more harm than good, and made the decision, with the full intent, and ability to deal with the fallout.
If a user any user flags a question or a mod comes across it - and on the whole, its better to not have around we may delete.
However, what we don't want is folks asking questions - with full awareness it could be downvoted, and deleting to avoid the consequences, especially on meta.
We even have a badge encouraging folks to engage constructively to unpopular questions - reversal.
So - in short, Moderators can make that decision, but in moderation. Users on the other hand, until they're aware of the full extent of their posts probably shouldn't. We're for most part accountable (I actually got that SF rep back!) and probably will very carefully discuss and weigh the pros and cons of the situation before deciding- even if its between drama now, and drama later.
If you're a user in good standing and can make a good arguement for changing the status of a post (either for deletion or undoing it) you can always flag for review, though in that particular case, with a less sensational post already in place, I'd recommend not bothering.
All of this boils down to one very important concept: Trust
In a nutshell, posters, by themselves, don't have any. They are afforded the luxury of being able to delete their own question, but only under strict criteria. Once those criteria are no longer met, it requires users that are trusted to use the system properly, and exercise their judgement to delete such posts. I'd argue that posters shouldn't have the ability to delete their post once it gets any answer, not just an upvoted answer.
Is it disrespectful to those that answer when their post is deleted? Well, it can be. Just like it can be argued that janitors cleaning up after someone tracks mud into a museum is disrespectful. I'd argue that leaving it in the hands of the asker, however, is far more problematic than giving that ability to the people the system trusts.
Janitors can get it wrong; nobody's perfect, after all. But just as they're trusted with that ability, they're also trusted with the reverse, to rectify their personal, or the community's, mistakes. That's what community moderation is all about; working together to make the site better, performed by those that the system trusts to do so.