I just finished reading Dylan Wiliam’s Embedded Formative Assessment. The book is wonderful, but one of the most enjoyable parts was the authors referenced by Wiliam. He mentioned Robert Pirsig, which prompted me to read Zen and the Art of Motorcycle Maintenance. He also wrote of John Wooden, Malcolm Gladwell, Barry Schwartz, and Marianne Williamson. You can be confident that an educational author who includes these people in his or her book is on to something.
Most interesting was Wiliam’s inclusion of psychologist Mihaly Csikszentmihalyi, who wrote Flow: The Psychology of Optimal Experience. Csikszentmihalyi describes ‘flow’ as being so absorbed in an activity, you’re not even thinking about it. It’s someone who’s attempting a task that’s interesting and challenging. Wiliam explains it this way:
When the level of challenge is low and the level of capability is high, the result is often boredom. When the level of challenge is high and the level of capability is low, the result is generally anxiety. When both are low, the result is apathy. However, when both capability and challenge are high, the result is ‘flow.’
That sweet spot of high capability and challenge is what all teachers should strive to implement in their classrooms. An example of this is Zone of Proximal Development (ZPD), which is used by STAR Reading and Accelerated Reader to help students pick books that are challenging, but not too challenging. A book within a student’s ZPD may not be easy, but the goal of finishing it is not out of reach. Likewise, the objectives a teacher introduces in class must mirror this type of ZPD criteria. Student buy-in is highest when the objective is challenging and the student believes he or she can learn it. When this occurs, classes hum along with the sounds of student inquiry and productivity.
RISE AND CONVERGE 
A peculiar phenomenon in special education is to intentionally strive to teach below student capability. This happens for a few reasons. First, teachers want to encourage success, which can be elusive for these students. Keeping expectations low can guarantee success. Second, teachers fear the possibility of a special education student not making progress. This can earn the scorn of parents and even result in a due process hearing if taken to its end. Third, sometimes class management is the most difficult hurdle with students in special education. Although giving work that is too easy could insult them, giving work that is too difficult will likely result in students becoming frustrated, tuning out, and acting up. To avoid this, many teachers go the route of easy and self-esteem boosting work. All of this is unfortunate, because ZPD really is a great way to encourage true growth.
Yes, all three of the examples you stated are big reasons why the special education population is taught with curriculum below their capabilities. Using a ZPD method is really difficult, and that’s why it’s important that teachers are effective at applying formative assessments so they know when to step on the gas and when to tap the breaks.