The reason that employers should be allowed to do this is that the employees they hire are the backbone of their business. If an employer hires a worker who does not work out, their business can be harmed and they can lose money. At worst, their business could have to close if they make a mistake at a critical time and in a critical position.
Of course, employers should not require applicants to take personality tests unless the tests are related to the job. If they do, they are wasting their money. They are also running the risk of rejecting a perfectly good employee because that person “failed” an irrelevant test.
If, however, the test is relevant, employers should be allowed to administer it. It is very difficult to determine, simply on the basis of a resume and an interview, if someone has the right personality for various kinds of jobs. Sometimes, the person’s performance on the job will be strongly impacted by their personality. In such cases, it is important to know what the person’s personality is like (in some objective way) before the employer makes the hiring decision.
Business owners have the right to do almost anything to ensure that the person they are hiring is a good fit for the job. A personality test can certainly be useful and is not something that should be so offensive to an applicant as to warrant prohibiting employers from using it.