My current employer has a load of mugs (coffee type, not other employees) which are printed with their logos and livery, they get given to customers and are handed out with other goodies at exhibitions, this is nothing unusual and I bet your employers do the same.
At one point I had a couple of boxes of them to distribute around the offices of the customers I work with as you can never get a mug when you need one, of course I kept a couple to take home with me too.
I noticed an odd thing the other day; if I'm working from home and have a cup of coffee I'll only use one of the work mugs, even if I need to wash it up first. Similarly, this morning I brewed a coffee and the only mug in the cupboard was a work one, I couldn't bring myself to use it and washed a different mug to use!