Companies don't have to accept jobs. But I think it is common courtesy to give a good reason for not accepting the work. But as others have said, you often don't hear back from them. They will come to do the quote, and then you never receive anything. Also if they thought you could be difficult to work for, this can be another reason for turning down work.

I have had contractors who were not prepared to do work due to the type of work, so it is common place. It does often depend on how busy the business is. They must be doing pretty well to turn down work though so, move onto another company. There are heaps of them. Best to get a contractor from word of mouth. I have never been let down by a word of mouth contractor. But I have been when searching for one from the yellow pages. Don't go for the one with the biggest advert.