Just been doing some research about all this - in Britain they call them DOGs! (digitally originated graphics) and there are several examples of them being removed due to viewer response.
In fact, in one case, BBC 2W, in November 2001, they were smart enough to remove it after realizing a DOG was generally hated.
There's a good number of pages on the topic at the DBA forums, including examples of letters people have already sent. You might like to include the ideas of the ABC being paid for by the taxpayer, and treating the audience as too dumb to realize what channel they're watching. Plus that the watermark is irritating and distracting from the program.