It's so the carrier doesn't waste hours every day scanning every letter for a service that only 0.05% of them need.
The charge for a return receipt is also higher and the USPS takes advantage of this just like all kinds of businesses have higher margins from certain products.
The many savvy people who take advantage of the lower rate for parcel delivery confirmation are like people who bring their own soda to work because it's cheaper then the vending machine.
TLDR: angry mob on the street probably wrong about finer details of world again.
Does delivery or signature confirmation (DoSC, I made this up) service cost money? Is it profitable? If the answer is yes and yes then charging for and scanning 95.95% needlessly DoSC items is good for the USPS and the 3/4 inch requirement is seemingly stupid. If the answer is yes and no then a silly requirement makes sense, but is still silly. If the answer is no and no then the service should probably cost money.
The charge for a return receipt is also higher and the USPS takes advantage of this just like all kinds of businesses have higher margins from certain products.
The many savvy people who take advantage of the lower rate for parcel delivery confirmation are like people who bring their own soda to work because it's cheaper then the vending machine.
TLDR: angry mob on the street probably wrong about finer details of world again.