DRM is a means of limiting the distribution of a purchased (or licensed) digital file by the owner (or licensee). Exclusively locking a subscription service to a platform is not DRM. Rather, it is a means of boosting the sale of the platform by offering additional platform-only services. We can discuss the harm and inconvenience that platform lock-in may cause. However, we should not confuse the issue with DRM. That will just inflame old passions, preventing someone from approaching this new distinct issue from a fresh perspective.
No doubt many people against DRM will also be against platform lock-in. Perhaps others may not. For instance, I am generally against DRM. I purchased a digital file; I would like to be free to make copies of it for my own use. However, with platform-based subscriptions, I just can't get all that upset about it. I don't own an Android device, so I won't subscribe to Google Play. Also, there are a wealth of quality subscription services out there that run on all of the popular platforms. So what's the big deal?