Apple developed the software, is footing the server, power, bandwidth, and ops bills for three months all for free. The labels are chipping in their content for three months.
Sounds like everyone is bringing something to the table. Apple is incurring real costs. The labels, not so much.
Three kinds of customers will try the service:
People who buy their music: these will either become long term subscribers or not like the service and buy the songs they want anyway. Perhaps slightly delayed.
People who currently freeload: no loss for the trial. Possible huge win if they subscribe.
People using another streaming service: this is where the labels might actually give something. If a customer cancels their existing service while trying Apple's that will reduce the label's take by up to ~25% for the year. Month alignment and a comparison period will both drop this.
I suspect the third category is not the target audience. A small loss on service switchers is worth the big win for the labels in converting freeloaders to customers.