I suspect the review where somebody claimed population was useless also missed this rule. The rule makes perfect sense to me, because otherwise population would be useless.
I like to think of the $ as your *capacity* to generate money, while the represents your *capacity* to serve/accomodate people. Both are critical to generating income. A casino with 2,000 slot machines will generate a lot less income than one with 1,000 slots and 2 restaurants!