I just created an account specifically to post a reply to this, in case someone else comes upon the same issue. I'm using a Samsung Galaxy SII Skyrocket on AT&T, if it matters.
The active/inactive calendars are defined by (I think) the calendar app on the phone, which is what the Simple Calendar Widget (and other widgets) draw from. Active means that it is being used, synced, etc. Inactive means that no matter however much you try to sync it, it's never going to be flagged as something to be updated and you will be banging your head against the wall.
Samsung, naturally, doesn't have anything in its flavor of Android to promote/demote calendars to active or inactive (at least that I've found; if it does it's HORRIBLY unintuitive).
But there are other calendar apps out there that might help you with that. I used Business Calendar Free, which upon startup gives you the option to promote/demote the calendars you want. You don't have to continue using the alternative calendar app after that, though I'm doing so because it offers quite a bit more functionality than the native calendar app.
I feel like I've successfully promoted/demoted calendars before without having to go through this shenanigans but I did recently receive an update which changed a few things. In any case, this might solve some headaches for you!