One philosophical reason (as well as a practical one), you can do more with a non-dedicated ereader device.
Not sure how you define a "non-dedicated ereader device" but the Kindle Fire, for example, is an Android device, so one can have more than one "ebook reader" on that Android device. For example "Amazon Kindle" is just one Android applet on the device, FBReader is another, the Silk Browser is yet another way to read ebooks, it also supports PDF via another applet, etc. So all these device manufacturers take a certain amount of steps to partially "lock down" their devices, and people find ways to work around those locks, without even bothering to break the locks. Only the most adventurous take full "jailbreak" access to the machines, because it is easy to "brick" one's device, and manufacturers have been known to retaliate on their next software release. Which is not even getting into the issue of DRM-cracking, which would be a more serious business. I don't jailbreak and I don't crack DRM -- life's too short to waste my time on this stuff. But I find it good fun to figure out ways to work around the restrictions the monopolists keep trying to build into their machines. Don't know why they don't realize that people are happy to buy their stuff -- IF they offer good quality good value at a good price in the first place.