What? Plenty of hotels let you pay cash. Or is that sarcasm?

They'll accept cash if you fork out a $300 deposit, and the people who are likely to both pay cash and trash a room rarely have that much extra money on hand.

A friend of mine works night audit in a rough part of the valley. As soon as he says the magic words 'we require a major credit card or a $300 deposit', they immediately hang up.

This heavily depends on the country, plenty of places in the US and other credit-card-loving countries are credit (and specifically credit, NOT debit) card only for this reason. Plenty of tourists from countries where such cards aren't as popular get bitten by this.

I've never been to a hotel that didn't require a valid credit card as a hold for security when checking in.

Sure you can pay in cash at the end if you want.

But you can't get a room in the first place without a credit card, even if they never charge it.

(I mean I'm sure there are some exceptions somewhere, but generally speaking, no, cash isn't sufficient.)