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because of all the regulations, which can only be enforced because the cabbies have something to lose
You think that the loss of the "Good Housekeeping Seal of Approval", or "UL Certification", or the like is nothing? Don't you think that potential customers would quickly learn to look for that emblem on a cab?
If there was no government interference, cabs still wouldn't
Nonsense. You're just making stuff up, without the slightest evidence. Do you really believe that cab customers would turn away paying customers just because they're not white? And even if they did, that would just open an opportunity for a minority-catering company to make a killing.