It's very clearly backed up by the history of how the GPL came to be in the first place. There was a printer driver that RMS wanted to change but couldn't. If

It's very clearly backed up by the history of how the GPL came to be in the first place. There was a printer driver that RMS wanted to change but couldn't. If the system he was working with had had code signing for printer drivers and refused to run with his modifications because of it, we'd have had the TiVo clause in the very first version of the GPL already.