No one was ever gonna prove that.
But why's that a problem? If you are working under a contract, and your friend works for another company, can't he talk to you about you joining him once your contract is up?
It's a problem because the NBA is not part of a free market economy where workers are free to go from one employer to another like that. Analogies like this one are faulty for that reason. NBA players are not regular employees, nor are they paid as if they are. There's no reason to think they ought to be treated as such.
Players getting together and deciding that they are going to play together -- especially if they decide they will play on a specific team -- prior to free agency and negotiations with team officials is a problem because it undermines the ability of GMs to build winning teams.
It's not really just the talking that is the main problem, though; it's the fact that they were able to get to unrestricted free agency and all sign with the same team after having decided that they would. It's the fact that all Miami had to do is clear enough cap space to sign them (I won't speculate as to whether Pat Riley was aware ahead of time that Wade, James, and Bosh had an agreement, though it wouldn't surprise me at all).
I want an NBA where great teams are built from the ground up by smart GMs and well-run organizations that are good at scouting, acquiring, and developing talent. I don't want an NBA where great teams are created when (as Rick Reilly puts it) the best kids on the playground decide to get together and beat up everybody else.
If a team wants to amass 3 superstars the way the Heat did, they should be prepared to trade away assets they've put together and developed over a long period of time (like the Celtics did).