Because "plain text" is a MAJOR issue.
If it's encrypted, then Bad Actor #1 (in this case, Adobe) steals all your personal info. All fine and dandy; you can sue them and get them to drop all that personal info, and life goes back to normal.
If it's plain-text, then opening, say, an OverDrive eBook while at Starbucks suddenly makes your entire eBook collection, including titles that have nothing to do with Adobe, available to anyone else listening in on the public WiFi.
so: outrage over the action in the first place, merited. Furor over not even taking pains to protect the data from other eyes, way more than merited.
Think about it like this: which is worse: the guy next door taking pictures through your windows night and day, or the guy next door taking pictures through your windows night and day and posting them all online? They're both bad, but the second is much worse.