This is not a bug, because making it so that you can post your question immediately upon writing it, even if reading it imposes undue effort on the community, is not one of the goals of Ask Ubuntu.
As you're probably aware, in English, most sentences should be capitalized. Essentially the only time a sentence should not be capitalized is when its first word is never capitalized. For example, the sentence "iPods are too expensive." doesn't start with a capital letter.
Most people who attempt to write English, even if it is not their native language and they are just beginning to learn it as a second language, are aware that it's expected for sentences to start with a capital letter and end in a period, question mark, or exclamation point.
When someone is aware of this but decides to write sloppily anyway, they are wasting our time. Sloppy writing is harder to understand, takes longer to understand, and takes time to fix. And the nature of our ongoing project demands that we actually fix it, at least when the question contributes something.
So there is a quality filter. The quality filter could tell you the exact minimum that you need to change to get past it. (It's not always just capitalization.) But why would that be good? Right now, when you are prevented from posting by the quality filter, you have to try to clean up your writing style and make it more clear and correct throughout. That might be a bad thing from your perspective in the sense that it means you had to spend more time. But it means we have to spend less time to help you.
Let me put that another way: it means we can help you even if we have less time to spend. Thus it means you are more likely to be helped. A higher quality question is far more likely to be answered than a similar, lower quality question, even when the differences in quality are limited to large-scale capitalization, spelling, and/or punctuation.
Therefore, in the context of the situation you have presented, the current behavior of the quality filter directly serves the interests of the site as a whole, and indirectly serves the interests even of users who must edit a draft because of it before they are able to submit it.
There is a potential, related problem that I think we should take more seriously, though. There might be some people who simply don't recognize any possible improvements that could be made to their initial draft. Then they would be unable to post.
While it's not worth it to change the way the quality filter works to transfer work from individual authors to the community at large, the quality filter may cause problems for people who are unable to get past it at all, and perhaps that should be investigated.