Cheaper rent sounds like a better argument to me. Why would casinos specifically target poor people? Is the fact that poor people desperately seek ways to earn money serves as such a strong incentive to locate most of the casinos nearby?
I think even considering that it would be irrational. The poor don’t have much to give to casinos in the first place.
I don't think it's about reducing the cost of rent as placing a casino in an unfavorable area carries a lot more risk.
Just like rich people, poor people are looking for ways to make money, but unlike rich people, they don't realize that it's almost impossible to make money in a casino. The financial situation of poor people does not allow them to use many of the investment tools that are available to rich people.
Gambling houses can only make decent profit from poors through lottery and scratch off games, because the tickets are cheap, gamblers don't need to occupy the place for too long, what generates a higher flux of customers and the profit is made by quantity of players instead of the quality of players.
But for traditional casinos houses it's not interesting to focus on poor gamblers, because they will occupy a slot machine or a cards table to make cheap bets, while a rich player could be sat in front of that same slot machine or table making more expensive bets during the same time period.
Also, while it's true some poor people think they can make profit from gambling and for that reason they spend more than they should, rich people also spend a lot of money in gambling, not exactly because they think it will return them profit frequently, but because they have a lot of money to expend in hobby activities like this without bad consequences later.