My question involves employment and labor law for the state of: Washington/Federal

I work for the Department of Defense (Army Civilian) in Washington State. About a year ago, our boss told us he was going to start placing us in job assignments based on the times we arrive for work. Although it is voluntary to come in early, it is clear that those who do come in are getting the easiest assignments and those who come in later (or on time) are getting stuck with the worst ones (no clear definition of what is meant by better or worse, but some positions are requested 1st on a daily basis). It has gotten to the point now where people are coming in an hour early to get these choice appointments. While those who come in on time are left with no choice and get stuck in the most unpopular positions. I should note that our start time is 4am, and so people are arriving at 3am. With some of us living further away than others, this would mean getting up at 2am to go to work and then waiting an hour for the bosses to arrive. Positions are based on the order in line...with the bosses not showing up until 4am. I have gone up the chain of command and spoke to union rep who both responded that it is voluntary. I just feel next they will be asking who is going to come wash their car or make them coffee. Is this legal????