I was on US from LHR-PHL, a rather smooth flight throughout. The pilot never turned the Seat Belt sign off throughout the entire flight of 8 hours. Often a FA would announce "The seat belt sign is on please remain in your seats while the seat belt sign is on". This is ridiculous. This is whey people do not respect the sign. The crew (including all the cockpit crew) used the lavatories often. Why would a Captain do this and then have the FA tell people to be seated? How ridiculous is it for them to think people can go 8 hours without using the lav's? I only see this on US.