Keep in mind that the .45 Colt has been around since the late 1800s and was originally loaded with black powder. SAAMI specs it at a pretty low pressure out of respect for the older guns that can't handle the more modern pressures. This shows in reloading manuals, too.
As for loading down the .44 Mag? I can't think of a reason not to as long as you're not loading squibs. Don't want a bullet stuck in the barrel, after all...