FBM Shipping discount – Unethical, smart, or both?

So I’ve noticed that Amazon is pretty persistent in ensuring the UPS ground is the cheapest shipping option.

For items that would normally be first class packages, Amazon usually has UPS ground at ten cents less than the USPS First Class rate.

No matter what the weight is under a pound, UPS prints out a one pound label.

What would stop someone from putting in a weight of 4 ounces (the low price limit for USPS), even if the weight was up to 16 ounces, in order to get Amazon to provide the lowest price UPS label since UPS always prints out 1 pound labels?

For example I have a 15oz box. If I put 15oz in there, USPS is at $5.27 and UPS is at $5.17

If I put 4oz as the weight I get the same UPS 1 pound label printed out, but since the USPS rate is $3.81, the UPS label only costs $3.71.

I haven’t taken advantage of this hiccup in the system, but I can’t see how one would be caught since they are printing out one pound labels no matter if they are 1oz or 15.9oz.