You aren't entitled to good errors from someone else's web app
This particular small rant starts with some tweets:
@liamosaur: Developers who respond to bad URLs with 302 redirects to a 200 page with error info instead of a proper 404 page should be shot into the sun
@_wirepair: as someone who does research for web app scanners, a million times this.
@thatcks: It sounds like web apps are exercising good security against your scanners & denying them information.
If you are scanning someone else's web application, you have absolutely no grounds to complain when it does things that you don't like. Sure, it would be convenient for you if the web app gave you all the clear, semantically transparent HTTP errors you could wish for that make your life easy, but whatever error messages it emits are almost by definition not for you. The developers of those web apps owe you exactly nothing; if anything, they owe you less than nothing. You get whatever answers they feel like giving you, because you are not their audience. If they go so far as to give you deliberately misleading and malicious HTTP replies, well, that's what you get for poking where you weren't invited.
(Google and Bing and so on may or may not be part of their audience, and if so they may give Google good errors and you not. Or they may confine their good errors to the URLs that Google is supposed to crawl.)
Good HTTP error responses (at least to the level of 404's instead of 302s to 200 pages) may serve the goals of the web app developers and their audience. Or they may not. For a user-facing web app that is not intended to be crawled by automation, 302s to selected 200 pages may be more user friendly (or simply easier) than straight up 404s. As a distant outside observer, you don't know and you have no grounds for claiming otherwise.
(There are all sorts of pragmatic and entirely rational reasons that developers might do things that you disagree with.)
It's probably the case that web app developers are better served over the long term by doing relatively proper HTTP error handling, with real 404s and so on (although I might not worry too much about the exact error codes). However this is merely a default recommendation that's intended to make the life of developers easier. It is not any sort of requirement and developers who deviate from it are not necessarily doing it wrong. They may well be making the correct decision for their environment (including ones to deliberately make your life harder).
(See also Who or what your website is for and more on HTTP errors, which comes at the general issue from another angle.)
PS: If you are scanning your own organization's web apps, with authorization, it may be worth a conversation with the developers about making the life of security people a little easier. But that's a different issue entirely; then 'our security people' are within the scope of who the web app is for.