Bill, they do do it inland too. I am 70 to 100 miles from the coast and I see SCE trucks washing insulators every so often. I imagine the nasty smoggy air is a contributor to insulator tracking.
Oh, one more thing. Transmission towers (out here at least) are not grounded in the way that we all think of it. They have static ground wires that run up high on the towers but if you look they are insulated from the tower. The towers themselves have some coiled copper in there footings but there is no bonding done from tower to tower. Granted at these voltages the earth can be a pretty good return though.
