I don't think it's anything to do with how prestigious a conference is that determines whether a paper has to be submitted in advance or not. I usually decide how important a conference is in my field by who the organisers are, the place it's being held, the keynote speakers, who's spoken at past conferences run by that organisation etc. I've never had to submit a paper in advance, but I definitely wouldn't say the conferences were low prestige either. Might it be related to the possibility of the event organisers publishing the proceedings or something? Your idea about emailing the organisers to check in advance is a good idea if you're worried.