Simple.. put some mpaa music with your drone footage, and then the MPAA has youtube activate ads... wonder how that would work out. Or still, if I take amateur footage, and TMZ wants to buy the copyright to that footage, can I sell the copyright? It was still an amateur, non-commercial flight... wouldn't copyright law (constitution) trump FAA regulation?

But there's an even weaker argument for the monetized video distinction than the flying with a friend distinction, because the former happens potentially long after the flight even occurred. It's entirely possible to not even intend to monetize a flight video while recording it, and in that case, it's impossible for the subsequent monetization to affect the flight itself in any way.

It is possible to not intend to monetize a private flight but in any case it will be illegal to accept payment for the flight in the future.

And that scenario (if the legal status changes retroactively) is equally ludicrous.

The legal status does not change retroactively. If the flight was done according to the rules set out for private flight then you, as a pilot, are prohibited to make any money from it (with some exceptions). Getting commercial license will not allow you to earn money on previous flights that were done as a private pilot.

I would imagine profiting from experimental aircraft is forbidden mostly for safetly concerns, since you probably wouldn't want someone making a fleet of aircraft out of their garage and routinely selling flights with them. Certainly that aspect doesn't apply to drones tho, since there's no inherint safety risk that I can see.

There is though.

The potential for monetary reward for drone flights fundamentally changes the risks a pilot is prepared to take. If the conditions are marginal or if there's more people around than I feel comfortable with, I pack my gear up and go home. If there was a significant amount of money at stake in "getting the shot", I'd be _very_ tempted to push the limits further than I do as a hobbyist. If there was a large amount of money involved, and the risks of getting caught were low enough, it'd be very tempting to break the law to get the money. I choose to stay right out of that game for those reasons - but you only need to look at the paparazzi industry to know that when the money is right, people are happy to do impolite, dangerous, immoral, and illegal things to "get the shot". Even as a hobbyist multicopter builder/flier I'd rather not see "my hobby" being used that way - at least not without some accountability and consequences for people who choose to behave like that.

Does legality really matter if you are the point where money is the dominating factor? Funny you mention paparazzi...

Yes, because legal penalty becomes an expected cost of the activity.