Then they should quit.
The fact that they don't, indicates that Starbucks, scheduling hassles and all, offers them better pay than any other job that they are likely to get.
One fact of life is that companies respond to profit and loss. When the expense of having to deal with continual turnover, because people keep quitting, starts affecting profitability, then they will address the issue. Not before.
Gosh, no wonder people like you fail to understand the lack of appeal of conservatives to the working underclass and why socialists like Sanders are so appealing to them.