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.
Personality tests are a sensitive issue. They are not necessarily of benefit only to the potential employer; they can be of benefit to the potential employee as well. If the applicant is advised that he is not temperamentally suited for the position for which he is applying, this can help him in understanding himself and in finding the right place to utilize his best talents. The potential employer may be primarily concerned about his own interests, and personality tests may be somewhat intrusive, but it is the potential employer who is bearing the expense of conducting the testing and evaluation, and if the applicant is wise he will take whatever advantage he can from the application process and the application experience. The employer doesn't want to put a square peg in a round hole, but the applicant shouldn't want to be a square peg in a round hole either. Many people end up in the wrong vocation and can suffer from it in a variety of ways all their lives.