Ask HN: Is there ever an excuse for browser-restricted websites?

Websites (generally some sort of form-filling/viewing site) that only work (or claim they only work) with a specific browser(s), specific browser versions, specific OSes, etc. Usually this is Internet Explorer or at least includes it.

Is there ever a good reason or excuse for this? I've only ever seen it in government/university/internal-company sites that (a) don't have a lot of development budget and (b) have a captive audience. Generally quality is pretty low as well. Is there actually a solid security advantage or something to e.g. only supporting IE7-10 on Windows XP, Vista, or 7?

(That last is an actual example of a "training program" site my current contracting agency is making me run through. I'm thinking of sending them a polite but stern email regarding the standards one expects in a company whose entire job is contracting coders and web designers.)