Earlier quoted context omitted.
Because the best way to get a user to "move in" is to make it easy for them to "move out". It's actually in Amazon's financial interest to make it easy for people to move off of their offerings.
Joel wrote about this back in 2000. Barriers to exits become their own barriers to entry. http://www.joelonsoftware.com/articles/fog0000000052.html
Sure, "barriers to exit become barriers to entry" is applicable in some contexts, and for some part of a potential market it will be the barrier that keeps those customers away. I wouldn't dispute that. I would dispute that "barriers to exit become barriers to entry" be treated as a universal maxim applicable along all businesses and products. It's just as much of a strategy mistake to make as assuming the barrier can't exist in the first place. It ought to be considered; not treated as a key issue until or unless the data suggest it should be.