So far hiring on psych test results isn't illegal discrimination -- I think. Discrimination isn't automatically illegal; it has to be illegal discrimination to be actionable. One is entitled not to hire someone who is mentally incapable of performing assigned tasks. One is entitled not to hire a 130-pound six-foot six guy as a bodyguard. Etc.
However there are two problems.
Practically speaking, if a company is using these tests, without a lawsuit you aren't going to find out why they make the decisions they do make using such tests -- and then they'll deny attaching much weight and claim a nonhiring was based on lack of qualifications, misfiled paperwork, whatever.
Second, from our point of view, some of these tests are better than a lot of people think at ringing a bell when someone is trying to steer the outcome. Or, much worse, the people selling the tests to employers say that's the case. And you'll never know.
Reverse the situation, though. If someone won't hire you on the results of a test showing you to be INTP, you probably don't want to work for those people. They're wrapped too tight and counting paper clips on their coffee breaks.
And face this, also: Bad job fits make us miserable and we're hard enough to be around when we're not miserable.
More and more I think an early finding of INTP should mean that person is exposed to entrepreneurial opportunities. When I was in high school, if someone had said "You know how when the teacher is wrong but you can't do anything about it because they have authority to teach errors, and you sit there and want to scream? Yeah, well, if you get a job, odds are someone like that teacher is going to be your boss. You'll work hard for years to get to be a boss so you don't have to chafe under the shortcomings of a boss. Guess what? If you get to the top of the pile in 30 years, you'll then find you chafe under the shortcomings of the people you supervise."
"So, you know what? There's this thing called small business ownership. Find a product or service, set up a system to sell it, work for yourself, kick yourself when it goes wrong, expect some of the stuff you try to not work out, but seriously consider preparing yourself for this, otherwise you will have to endure fools your entire life."
I really wish someone had said that to me.
Anyway, be careful in any environment in which a fuzzy-seeming test is required. Those things are a wild joker added by a potential employer to the deck of cards you bring to the game.