People who can't afford cars still buy other things, therefore 'such people' also contribute, albeit less in absolute values. The amount of a purchase of a big

People who can't afford cars still buy other things, therefore 'such people' also contribute, albeit less in absolute values. The amount of a purchase of a big spender is greater, therefore he is worth more to an advertiser. This means each person financial contribution to a ad supported service is proportional to its spending (or more precisely to his ad motivated spending). That is not equal to being proportional to each viewers income, but these amounts are probably correlated. Also, as lmm commented, the more people using some services, the more value they provide, so having a fixed price would be a net loss for everyone.