counterfeiting doesn’t stop being a crime because the fake bills suck.

It stops being an effective crime that is significant enough to warrant disproportionate intervention with printer design. Someone who would use a SOHO printer to counterfeit banknotes isn’t going to the trouble of making paper that integrates colored fibers into the paper. Maybe lousy counterfeits will fool some low-grade vending machines and some kids will loot some candy bars. For me that’s not justification for fingerprinting every single printed page using ink that the customers pay for.

Also, if appeasing the Secret Service isn’t the real reason, why aren’t black and white printers printing gray dot codes?

A gray dot is harder to hide than a yellow one.

BTW, it’s worth noting that the whole industry of counterfeiting yields less counterfeit money than what the secret service spends on controlling it. It’s security theatre for the sake of reputation and respect for the currency.