Apple do pay for all the costs incurred by the infrastructure, hosting, and delivery of the artists content. Apple invest in the platform on behalf of, yes themselves, but also the artists. If this cost is not shared, then it doesn't seem unreasonable to me that the '3 month trial' should serve as an investment in the platform, equally by the artists that will serve to generate them revenue in the long term.
Does Apple have to compensate the artists for their own out of pocket costs to be professional musicians, like equipment and recording sessions etc? No. So why should the musicians have to compensate Apple for what essentially amounts to the same thing? Apple doesn't make it's money from the server farms or software development, and the musicians don't make their money from buying equipment or spending time in the recording studio. You should pay for what makes someone money, and that's what royalties are...so it makes perfect sense for Apple to change the policy.