The employees should quit if they feel that strongly about it. That’s one of the great things about America, you don’t have to work at a job that you don’t like. Pretty simple huh?
They can quit, but they aren’t likely to find work as easily as Amazon’s staffing agency can find a replacement or for Amazon to replace the staffing agency.
That is known to the individuals working at Amazon, the staffing agency, and Amazon itself. Absent inordinately good conditions, that can set up strange incentives to treat the staff with a certain degree of contempt and distrust.
For what incentives exist to set things up this way, I would be surprised to hear of genuine (read: not PR-driven) examples to the contrary. That is, somewhere that values their employees and agency help in ways that positively incentivize someone to want to work there (read: in ways other than it being the only thing available).
There is one other solution. Vote in a labor union such as the UAW, SEIU, or Teamsters and then negotiate compensation for standing in line. A union shop would end Amazon as we know it; management does too.
Management has responsibilities for security, but how long is reasonable, 10 minutes yes; 1 hour no. There should be some easy solutions to this problem.