Well... my first Star Trek eBook was a DRM-protected Microsoft Reader format eBook. Even though I paid full price for it, at the time, it can no longer be read, because Microsoft Reader no longer exists and the activation servers for those books no longer exist. (It's actually hilarious: I bought it on Amazon, before Kindle was a thing.) While the format can be converted, the DRM encrypts it, which is, at minimum, annoying to strip off. DRM also limits what readers you can use for the eBook: Our Remarkable tablet is an excellent eReader, but it doesn't support DRM, so eBooks so encumbered aren't compatible with it.
In short: DRM is what differentiates an eBook from being a rental with an unpredictable expiration date or a true purchase. We've very lucky that, with a few exceptions, Star Trek books are sold DRM-free, despite the fact that most of Simon & Schuster's other imprints are sold with DRM.