Only full-time hourly workers who have been with the company for three consecutive years are eligible for the training benefits. It wasn't immediately clear what proportion of its fulfillment center staff that would include, and Amazon declined to say.

Amazon in November 2011 that it had more than 15,000 full-time permanent employees in its 34 U.S. warehouses. But an undisclosed number of fulfillment center workers are temporary or part time, and therefore do not appear to meet the Career Choice eligibility requirement.

During the winter holiday season, for example, Amazon brings in "many more people" to staff fulfillment centers, Bezos said in his letter. The company's also solicits temporary workers.

Still, Amazon's efforts appear to be softening some of its critics. Working Washington, a Seattle-based labor rights group, welcomed Monday's announcement.

"We are glad to see Amazon responding to public concerns and beginning to take steps to improve conditions for workers in their warehouses ... Amazon certainly hasn't solved every problem at their warehouses, but this is a good step," Sage Wilson, a Working Washington representative, said in an email.