Your anecdotal evidence does not equal "reality of the market". It just demonstrates their particular situation. You can't draw an inference that their underemployment is due to the nature of their degrees. 
There is no guarantee that your friends wouldn't be even more in debt and still working at Starbucks if they had STEM degrees. Getting hired is about more than just a piece of paper. It's not enough to simply be able to graduate. You have to be able to market yourself effectively too. I submit that your friends are lacking in concrete job searching skills and have settled for the easiest job to land. No one is going to hand them a job on a plate. They need to get creative in order to demonstrate their abilities and they need to grow their networks. Alternatively they could do a 1 year post-graduate diploma program at a college to acquire more practical skills. The reality is that not every graduate is attractive to employers regardless of the degree they hold.