But if you think about it... she wasn't getting anything from Apple Music subscriptions before it was a thing. In 3 months time, she will start getting money from Apple Music subscriptions.
How is it putting people out of jobs when they got 0 before and will get 0 for free months and then something?
Swift does get paid from streaming services. Folks are going to switch over to Apple Music during the trial service so there will be less streaming on, for example, Pandora, where Swift is paid. I as customer will be getting Swift's songs streamed to me, but she will not get paid for three months. It is as simple as that.
She clearly is ready to support Apple Music, for various reasons, not the least of which she is super rich and can easily take the hit. But she is speaking up for other folks who are living closer to paycheck to paycheck and who are going to see smaller checks from iTunes and other streaming services for the next three months.
Also Apple is using the content to build Apple's service. Why does the very rich Apple company need that content donated for three months? You haven't addressed that point. Though in some ways the answer is easy, it saves Apple money to get the content donated for free for a few months. And also most of the content comes from the big labels and this is indirectly how Apple is getting the big labels to chip in some cost for building this service out.