It's a safety measure. When it comes to regulating aviation, in general, the FAA is concerned with protecting the public. The "public" doesn't generally have the knowledge to evaluate a pilot, aircraft, or the operation of an aviation venture. So, the FAA puts rules around these things. The private pilot is held to a much lower standard of skill than a commercial or airline pilot. But, that means that the FAA doesn't trust them to fly around the general public.

You can take your friend for a spin in your plane if you want, or go screw around and kill yourself, but you cannot "hold out" your operation as an air taxi or airline to the general public, and you can't make money off of it in any situation.

A commercial pilot has to undergo much more training in operating an aircraft safely. This means the FAA allows them to get paid to be a pilot - they could be hired to fly someone around in that person's plane. But the commercial license does not really train them in running a safe airline, so the FAA does not allow them to use their own plane to run an airline.

EDIT: To word it differently, the opportunity to get paid increases the likelihood that someone will push limits or take unsafe risks. If you aren't under pressure to make your paycheck, you're less likely to take your passengers into marginal weather. (One of the most dangerous occupations in aviation is medivac/aviation EMS. There, the pressure isn't generally monetary but moral: you want to help a sick patient, so you take more risks.)

I'm not totally sure what the GP meant, but I think it has to do with the owning/operating organization of the plane not being a qualified (Part 121/135) company for commercial operations.

A grandfather-in-law owned his own small business (a civil engineering firm), and had a plane that he flew to get to meetings/job sites across the Midwest. He could fly company employees just fine- and the company could reimburse him for the flight expense, and since it was not for the public that was fine. He could fly his family or friends on his own dime just fine. But if a family member or friend not working for the company tried to compensate him for the costs, then it is a question of "is his company actually an unlicensed airline?" and now we're getting into territory where it gets complicated. The FAA heavily regulates airlines, which is a major reason they are so safe. But there has to be a lower bound on what gets regulated, and avoiding that is what I think that GP is referencing.

It's worse than that. It's due to some weakening in the checks and balances system of US federal regulations, as well as a hole in the system.

Historically, laws were written by the legislative branch (congress), enforced by the executive branch (the office of the president), and overseen by the judicial branch (federal courts). When oversight works, all three have to agree for an enforcement action to take place.

In somewhat recent history, congress has been passing regulations much less, instead passing authorizations for the executive branch to write their own regulations. On top of this, the judicial branch had, until last year, allowed executive organizations to use their own judges, without juries, and follow their own interpretation of laws and regulations, when it conflicted with a literal interpretation. See https://en.wikipedia.org/wiki/SEC_v._Jarkesy and https://en.wikipedia.org/wiki/Loper_Bright_Enterprises_v._Ra... for the recent rulings overturning those precedents.

The effects of those last two cases are still working their way through the relevant bureaucracies, so until then, the FAA will keep with their current interpretation of gaining experience as a form of commercial income (https://www.faa.gov/media/15611) and having a reputation as form of advertising (https://www.faa.gov/documentLibrary/media/Advisory_Circular/...).

Even then, there's a hole in the system that especially impacts overly broad definitions of commercial activity, which as far as I'm aware, no other executive organization has taken anywhere close to the extreme of the FAA.

That hole is that even with the precedent of the recent supreme court rulings, they only take effect when something goes to the courts. With real commercial activity, this happens regularly and often preemptively, because the costs and resources needed to take something to court are easily recovered by the extra income or savings of a ruling that follows proper checks and balances.

On the other hand, there is no incentive to spend the money and resources needed to regain rights for non-commercial activities, like carpooling (or airplanepooling) where no one is earning anything.

> I'd like to understand why that regulation was written before getting rid of it.

It's simple. Getting a commercial pilot license is a much more involved process than getting a private pilot license.

A private pilot needs just around 30 hours of flight time to get a license. A commercial pilot needs at least 250 hours and a medical certificate that needs to be renewed periodically.