I'm not the parent poster, but I had a similar reaction. It's not the DRM specifically which is the problem.
If I go with Amazon I can read any e-book in my web browser, on my e-reader, or probably in an app. I can access it anywhere. Yes, it has DRM on most of its books, but that doesn't introduce a practical limitation because I can read the book on any device - laptop, tablet, e-reader, etc.
With Bookshop.org they only allow readers to view their books in their proprietary mobile app or in a web browser. This pretty much kills the option of reading on an e-reader. Which means the device I specifically bought for reading digital books can't be used with their service.
Now _some_ of the Bookshop.org titles do not have DRM (some Amazon books don't either), but there is no filter for finding them and no option for removing it. Which means their service is effectively incompatible with e-readers. While Amazon's service, for all its many faults, works with e-readers.