Froyo Sound Management
Silent Mode definitely isn't supposed to mute media or alarms.  Both of those are controlled by you, unlike calls and other notifications that are caused by external events.  You wouldn't want to miss your alarm just because you forgot your phone was on silent :) but it's definitely strange that it disables the volume controls entirely, that must be a Motorola thing.

You could try the Audio Manager app for better control over the settings.  It doesn't sync the media volume to the ringer volume, but you can use profiles to quickly set up one where both are muted and then it's easy to switch with a click or two.  It also manages Alarms, Notifications, in-call volume, and "System Volume" (probably the noises you can enable for when you click things and so on).