There’s no conspiracy here, any more than there’s a conspiracy behind the large discount you can get if you buy a year’s supply of toilet paper at once.
It is true, however, that wealthy people carry much less risk for them so they may have similar expected values (this is just a guess)
If one poor person is paying, say, 11% interest on their $500 balance for a year, then the company makes a few hundred bucks on the interest. If a wealthier person spends $50k (admittedly random number I made up, its going to vary vastly for different card-holders, and probably in the millions for many) with their card over the course of the year but pays it all off immediately, they've made between 1k and 1.5k risk-free on the 2-3% merchant fees, and the customer will probably continue to use that card.
Like the article says, the fees make the cards a lot of money.
That person putting $500 per month on the card likely isn't poor either unless they are paying most of their expenses with it and paying the card off each month. Again, they get some fees. With a limit high enough for $500 a month without paying it off, they aren't as high risk - after all, that is at least a $6,000 limit. That's a full 20% of a 30k/year income, after all.
These folks are in a middle ground. The company can make some of their money off of transaction fees, but will try to make up for it through interest rates, which won't be all that horrible.
The actual poor person has a credit limit of $500-$1000. They aren't spending enough for the company to make much from fees, so they try to make it up in interest.