>The Sea Launch system is designed to take advantage of physics that allow a rocket launched from the equator to carry a heavier payload into orbit than it could if the launch point was anywhere else on the Earth's surface.

To get to a circular orbit you need to do ~7.9 KPS. Launching from the equator means that you take advantage of the earth's rotational speed of ~1KPS lowering amount of energy you need to expend to get to orbit.

They knew about this decades ago, that why everybody involved in launching has their rocket launch sites as close to the equator as possible.

It's not just the boost from equatorial rotation velocity. These satellites have to go into zero-inclination orbits, because otherwise "Geostationary" really means that their latitude oscillates up and down across the Equator. If you launch from 28N (South Florida), it requires a huge amount of extra energy to shed that unwanted orbital inclination. From the Equator, you are already at the desired inclination.

No, they're not all as close to the equator as possible. Plesetsk for example is at 62.8 N, which is actually an advantage for many desirable orbits, as noted here :

http://liftoff.msfc.nasa.gov/rsa/pads.html

although it's not a very recent webpage.