That pretty much ruins it for me right there. These guys talk out of their aces more than anyone, so the entire notion could have been fabricated. I can't see any reason that the NFLPA wouldn't want to allow teams to carryover - because teams aren't ALWAYS going to spend to the limit. And when they don't, it's way better for the players to roll it over rather than have it be use it or lose it.
The NFLPA is against LTBEs because they allow teams to get over the salary floor. There are certainly teams that have violated the minimum per-team salary requirement, but for LTBEs. They probably have no problem with teams carrying a couple of million over from one year to the next. But teams that are carrying over tens of millions, that is going to be a big issue, because it creates the ability for individual cheapskate teams to underspend on player contracts.
There's nothing unethical about the LTBE loophole. It's simply a means of fowarding leftover cap room from one year to the next. It's been done for 10 years now, the league is well aware of it, as are all teams. And there's nothing really artificial about it. The team had remaining room at the end of one season. It should have every right to carry over that cap room to the next year.
If anything, one could argue that the LTBE loophole should be made unnecessary by the league automatically forwarding remaining cap room from one year to the next. But where you get a question of ethics from teams using this loophole, I don't know. It would be like arguing that it's unethical for a taxpayer to take every deduction they're legally entitled to.
I don't see it that way. Let's start with the name of the provision itself - likely to be earned. Writing a contract stipulating that your 340 lb. starting DT is likely to participate in 80% of your offensive snaps (or whatever) is inherently specious, and clearly not in keeping with the original intent of that clause.
If the league truly wanted teams to have the right to carry over their unused cap room, they'd simply transfer it as you said without the need for this type of bogus contracting. No mess, no hassle. But they haven't done that, even though that would clearly meet with no objections from the union.
Further, if there are no ethical issues to be considered here, then why are there teams eschewing this method and putting themselves at a competitive disadvantage?? I think you know the answer.
As for your tax metaphor, I'd say your painting with way too broad a brush there. Ethics aside, filers get stung by the IRS all the time for pushing the envelope of the tax code.