They do get enforced. Try taking a Priority Mail box and telling the postal employee you only want to pay for first class postage.
Let's examine this, how exactly does this work? I know when I bring in a cardboard box of my own, with--for example--a Hess Truck inside, there's no way that box is going to weigh in at first class. At best it will qualify for parcel post, which in most cases isn't any better than the Priority Rate. That leaves the Media Mail option, which is pretty much screwing over the Post Office no matter what kind of box you use.
I'll tell you another reason that box will never weigh in at first class. Whenever you take a package into the Post Office to be weighed, they're counting the packing materials against you. In fact the only way around this would be for a seller to make up their own pre-paid packing slip, print it off online, factoring in the weight of the item and not the packaging materials. What's the take-home message here? It looks to me that the electronic printing of labels hurts the USPS more than anything else.
The reason these "regs" aren't enforced like you claim is because douchebags are deceiving the post office in an attempt to save money but in actuality are STEALING money from the federal government. In the grandscheme of things it is not different then cheating on your taxes and not getting caught. There aren't enough IRS employees to audit every return just like there aren't enough postal employees to open every MM package to make sure it contains only qualifying contents or to tear way the brown kraft paper on each box to make sure it's not a priority box underneath.
Get real, if the USPS was bleeding out money on this they would open up every media mail package and slap a fine on each recipient (pay or you don't get your mail delivered). This theft argument gets old because for one, plenty of CAGs have asked their postmasters about the Media Mail question only to find that they don't give a shit.
Two, I have the personal experience of paying out the ass for USPS parcel post and extra services; plus I see the overpriced packing material they sell in their stores--which all in all makes me less than sympathetic towards their terrible bouts of complacency when it comes to not enforcing their own policies.
And as I've said before, I would love to see some figures on the subsidies offered on bulk mail vs. the losses from flat rate boxes, because I'm thinking the largest source of theft from the USPS is LEGAL and CORPORATE.