I agree it's frustrating - and I've been on both sides of that fence - but still... it always puzzles me when someone seems to get worked up over the fact they didn't seem to get the callback they expected even though they had been told they were probably good enough. Why do people simply forget about the possibility that someone even better showed up right after them and got the spot instead, and rightfully so? It doesn't (necessarily) mean you were bad, or that they lied to you, or anything of the sort. It might just mean they had other candidates who fared even better.

Also, in these cases, I'd recommend you simply call back yourself. Think of it as the opposite of the judicial system (for the US and most of Europe, at least): the burden of proof is on the prosecution. In this case, it's on the job-seeker. You're the one who has to show you want the job and that you're the best fit. The company's role is to find a good fit (and to do so by using their resources in a reasonable way).

That being said, I wouldn't advise on doing "homework" assignments that are exceedingly long.

For the record, when I am on the recruiting side of the fence, I always do a phone screening first, with live coding in a shared editor. Only then might remote homework or on site coding come up, and I always tried to make sure the effort is minimal. It's hard to find something that fits all people and minds though. Try to design a recruiting scheme, and let me know how it works out and if it doesn't piss anyone off. :)

The courtesy thing is another issue entirely, of course, and obviously a lot less defendable...