I think it has a great deal to do with culture. Buying Macs for its employees means the company isn't making decisions solely based on cost - it means quality of the tool is also an important consideration. It indicates that might extend to other stuff. Are they buying the cheap office chairs, the best ergonomic chairs? Will they pay for an expensive standing desk? Are they willing to spend money on employee outtings? How willing a company is to open their wallet for the comfort and convenience of employees is a big part of culture.
That culture isn't necessary good for everyone. I know plenty of people who thing that kind of stuff is frivolous and useless and they would rather have higher salaries or stock options than expensive company culture.
Further, as difficult as it is to quantify, there is definitely a "type" of person who prefers Macs, and thus the leadership of that company or the people in charge of making purchasing decisions are that type.