They're employees. The employer pays them for their time and can demand that they attend whatever activities the employer chooses, from work training sessions to anti-harassment training sessions to sessions talking about corporate culture to sessions expressing the employer's views on topics like unionization.
If you want, you can hire someone to listen to you talk 8 hours a day on whatever topic you choose--there's no law against it.
If you want, you can hire someone to listen to you talk 8 hours a day on whatever topic you choose--there's no law against it.