Distasteful, or maybe imprudent, sure, but I don't think it's unethical. When I signed up for Gmail, ads weren't part of the bargain.

"Gmail is built on the idea that email can be more intuitive, efficient, and useful. And maybe even fun. After all, Gmail has: Lots of space. Over 7538.321890 megabytes (and counting) of free storage."

If just using the service obliges me to view the ads, doesn't it also oblige me to click on the ads? If I don't, I'm lowering the efficiency of their ad network, and thus the value of their advertising product. Maybe I'm even obliged to buy some of the stuff advertised?

That seems crazy to me. They didn't mention the ads when I signed up, so I just don't care. I don't owe them my attention any more than they owe me customer support.