Nobody is entitled to be paid to do what they choose. If you want to do something for your own personal reasons, do it off the clock.
And others are entitled not to pay you to do things they don't want you to do.
Nobody is forced to do things they don't want.
Nobody is forced to pay money to others.
But employment is an agreement between people. The employer agrees to pay, and the other agrees to follow directions of the employer (within the limits of their specific agreement).
If you go to a restaurant and order a burger, and they instead bring you a cake, you'd be well within your rights to refuse payment and complain. The fact that the chef wanted to make a cake is irrelevant, because the chef is being paid by you. (In contrast, if you were to go to friend's house, and he gave you a cake, you could not complain because you're not paying for it.)