Actually, I run about $3K/month on my Amex and pay it completely off, too. So, It isn't that I am not using it, nor suffering from a shrinking available balance (took me <90 seconds three weeks ago to move it from $9K to $25K to outfit the home theater electronics).
Then again, I do wonder if household income and credit worthiness might be played into this. Also, I have multiple Amex accounts. For instance, I have another that I use only for online purchases (<$500), maintain no balance, and its available limit hasn't gone down in the 7-8 years I have had it.
As such, I am not seeing what you are suggesting.
With a $500 card as it's original limit, I don't think you have many real cards.
Most cards do have a bottom limit for the card program, I can't see AMEX having a below $500 tier.
Household income isn't really a factor, your CC companies don't really know that.
What becomes a factor is utilization and some have said spending habits...it's a row of dominoes though once a few key company's yank their cards or reduce their limits.
Lets just say you had a nice card with about $20k available on it and then a mix of some store cards with $500-1000 limits you picked up for discounts, and then a handful random cards you rack up and pay off each month or two.
Say you hit a bad month and although your $20k card was just used it's always fully paid off, your other cards are all almost 100% full. Your big creditor is doing a review of their books and see your huge credit limit as only touched once and then fully paid off. To them that's a lot of exposure and they aren't making interest. Since you $20k is usually empty, you decide to make your minimum payments to the other creditors and a big lump sum to the $20k card bringing it down to a few thousand of payoff.
While in the process of closing this account your creditor sees it's now got a balance. They don't see any pending activity so they drop your limit to your balance.
You just went 100% utilized basically. Your other cards now see this and calculate their exposure. The lower limit cards may be at the program bottom so they jack the rate up to the limits allowed. You credit score now is dropping even more because since the interest is greater you are paying it off slower and your utilization is staying higher a lower time.