The unethical part comes in when you're purposely padding your estimates, delaying response, feigning work load, or profiting from a side business. The last one depends heavily on your work agreement, state law, and how the company chooses to interpret things. I know of a guy who was outed by coworkers for doing side work, but after review, management decided it was fine as long as he wasn't using the work phone for side-work calls.

Of course, nothing is black and white, and rationalization is just as strong an intoxicant as overreaching corporate policies are.