Apress is a freaking publisher - shouldn't they be expected to be at least minimally professional? Whatever the initial agreement e.g. regarding advances was, the fact remains that Apress did not pay the author until several months after they had completed the work despite several months of inquiries, reminders and so on.

Either Apress is just plain unprofessional (e.g. unwilling/extraordinarily slow) or as you said they are doing this in order to put themselves in a better negotiating position.

I had a similar experience with Apress while working with them as a technical reviewer. They printed the book without asking me for my bio, so I didn't get my five lines of text credits for the work I did. It wouldn't have cost them anything to do that, but they nevertheless screwed it up.

Needless to say, I figured it's way too much of a hassle to deal with them when they approached me to write an actual book, since I want to spend my time writing rather than following up to make sure my publisher does what they are supposed to do.

Yes, with a contract you have better legal recourse. But I'd prefer to work with people I don't have to sue in order to get paid, esp. when the sums involved are tiny.