I'm thinking of my last day working at a major coffee store. We made every drink decaf and zero people noticed
Is there a reason you made everyone's coffee decaf?
I like having caffeine free coffee but people are always mad when I give it to them. I just had enough of it.
It's not okay to lie to someone when they pay for a product. Someone may have wanted the caffeine in order to get them through a long day of work. Their performance could have suffered because of your lying to them.
It's fine. They should mind their own business.
I don't think it is fine because this is something that they paid for. It is wrong to lie to someone about the product that they are getting. You would be upset too if you didn't get what you paid for.