At my job the office workers get 5 sick days a year and sick leave. But us "slaves" out in the plant, where it is the Sahara desert in the summer and the cold Antarctica in the winter get no such privilege.

So is this legal to have one set of workers with sick days and the other set not?

Yes, it is legal to have a different set of benefits for different "classes" of employees, as long as Title VII is not violated (race, religion, national origin etc.). In fact, it's not even all that uncommon.