The numbers alone don't really tell you anything, even about how they treat their customers. At the very least, look at the content of the complaints, and how they were resolved. Then, use that as a basis for more research, if needed.
Some good examples can be found in this article. In the opening, it specifically talks about the interaction between Southwest Airlines and a customer. More generally, it talks about how the author feels it's important to stand behind one's employees than to appease customers (when the employees have done the right things, of course).
In the Southwest example, it's quite likely that the customer may have filed one or more complaints with the BBB. Likewise, customers who are jerks, and then "fired" from the company, may file complaints as well. These probably aren't valid complaints, and even if they are, it doesn't accurately reflect on how the company treats its employees. In fact, using your method of judging, it actually shows the complete opposite from reality - the company pissed off one or more customers because they defended their employees.
If you look at the content of the complaints, you can get a general idea of the nature of the complaint, and whether employee treatment may have even factored at all into the equation. It will also help you better gauge the company's overall ethics, especially if there are themes to the complaints (for example, I probably wouldn't want to work for a company that has a high proportion of complaints about breach of contract on the company's part).
Even then, it's not necessarily going to be an indicator of how the company treats its employees. It might give you leads to follow up on, but your best bet will be to talk to people who actually work(ed) for the company. Very often, word travels about who treats their employees badly and who treats them well.