metered exposures longer than 1sec

on my s45, the metering modes (auto, p, av, tv)
seem to never set a time longer than 1second.
I can get a correct exposure by going into
Tv and setting a long exposure time, then
half pressing the shutter, hitting the Exp Lock button
and using the selector to change among equivalent exposures. But this is unavailable in the automatic
modes, and I don't understand what makes 1sec
the cutoff, nor do I see this documented anywhere.
anyone know about this?

This is clearly explained in the manual! This is so that people who don't know how to use M(anual Mode) wouldn't get noise and hot pixels over the top. You can get exposires of 15 sec or shorter if you switch to manual mode. 15 sec is probably so that CCD doesn't over heat.

Hope this helps.