It kind of sounds like the place was packed, and there was no more availability for walk ups.
There was probably a long impatient line of people waiting to check in early.
Since nobody was being seated earlier than their ADR times, I bet a host or hostess walked out to the line and asked that folks wait until their ADR time to check in at the podium.
This would reduce the line for people who aren't getting seated yet anyway, and maybe cut down on a little of the impatience folks were experiencing.
I bet this would have been a great solution unless people start complaining that "The rules say I can check in 15 minutes early.....etc"
Unfortunately, this didn't work for you, because instead of following a simple instruction you got mad and repeatedly attempted to check in early.
Ya know, people aren't ALWAYS out to get you.
FYI, I'm not paranoid at all, and I don't think people are out to get me ever - but since you didn't quote in your reply, I'm not sure if you're referring to me or someone else that replied in this thread. This isn't really about me at all anyway - this is about 50s PTC going against the grain of what DRC agents have told people forever and what we've always done at WDW restaurants.
However, I will respond to the points in your comment:
1. It was packed, but they were still taking walk-ups, and quoting them 20-25 minute wait times, despite telling people with ADRs that it would be 30 minutes past their ADR time (and was 40 mins in our case)
2. There was a medium line of people waiting to check in (maybe five or six parties total), but nobody was complaining or being impatient, at least that I heard. I'm used to waiting to check in if we're near the dinner rush, and I only waited a few minutes to get to the check in desk, so it was actually moving rather well.
3. No one came out to walk the line during the entire hour that I was waiting outside the restaurant, though there were always two people at the check-in desk, and either two or three behind the other counter to the left of the check-in desk.
4. Which simple instruction am I supposed to follow? The "be sure to arrive at least fifteen minutes ahead of your reservation time to check in" that Disney has told me over the phone for more than a hundred reservations over the last 12 years, or the snippety "you can't check in, come back later" that a rude CM told me in this one instance?
5. I was mad, that's absolutely true - I choose to spend my vacation dollars at WDW at least partially because they understand service and treating people like guests, not customers. They've almost always hit the mark in the past, so this rude CM stuck out like a sore thumb.