I don't really agree with this because it opens up the whole hardware warranty issue once the case is opened but I do see your point like someone trying to plug a USB device in the TTL header
They did change their stance though now and are saying that even flashing 3rd party firmware voids the warranty.
**While Linksys fully embraces the open source community and is providing open source use capabilities, they do not offer technical support on using open source firmware. Installing 3rd party firmware is done at your own risk and replacing factory-installed firmware with OpenWRT firmware will void your warranty. Please always seek the advice of more experienced users via open source community forums if you’re unsure.