Unfortunately slackness, laziness and shoddiness are all too prevalent in every aspect of work and society. But whereas established trades usually have some kind of official certification and quality assurance scheme, web design has nothing of the sort. And most customers don't know enough about it to know when they're being sold a pup - at least, not until long after the event, by which time it's too late.
Yes, it comes down to business ethics. Are you prepared to rip someone off by doing a half-arsed job? Are you even aware that you're doing a half-arsed job? I'm sure there are plenty of 'professional' designers out there churning out utter crud without the faintest idea that they are doing anything that isn't of the highest quality.
While you might think that validation errors like missing alt text or unencoded ampersands are trivial, they can have more serious repercussions and they are so quick and easy to get right that there's no excuse. It is like not bothering to run an automated spell-checker on a document. Yes, we all make mistakes, and the occasional one slips through the net no matter how careful we are, but you can usually tell the difference between a one-off mistake and a designer who doesn't care about standards or validation.