I saw a Biography Channel show on him.
According to it, Beethoven added the "von" himself to sound more aristocratic. It came out of a court case in which he was trying to gain custody of his nephew from the boy's widowed mother. (
Casper was his father, by the way.)
The case was disastrous on many levels because public opinion was against him breaking up the mother-son bond. And when it came out that he was putting on airs of aristocracy with the articifial "von" it was just another thing to knock his reputation. (Beethoven had been a supporter of increased democracy and publicly supported
Napoleon when he first came to power in
France. The "von" looked hypocritical.)
I left off the "von" just because that's how I normally hear people talk about him. But I remember the Biography Channel show and just thought I'd pass it along. The show wasn't all negative aspects of him. It's worth seeing / renting.