Amazon's choice: (1) host Wikileaks and risk getting shut down, or (2) drop Wikileaks, get boycotted. If choice (2) is as bad as choice (1), something I guess the boycotters hope for, then Amazon's choice is a Morton's fork: two equally bad outcomes. Why don't the boycotters stick their Morton's fork where it belongs: into those who wrote and enforce the laws they object to?
Because Amazon does not appear to have been compelled by law, so there's no law to change - just a vague sense of not wanting to be on the wrong side of powerful people. This is how the mafia works. It shouldn't be how the government works.
The goal is to make 2 worse than 1 so that future companies, when faced with similar government pressure, will make a different choice.
As EFF says "Other Internet intermediaries should now expect to receive a phone call when some other member of Congress is unhappy with speech they are hosting. After all, it worked on Amazon."
I think a whole lot of people would react to 'those who wrote and enforce the laws' when scenario 1 was picked. Shutting down Amazon sounds a bit to extreme imo.