But that's not the penalty that the Edo specified for the crime Wesley committed. When Starfleet sets foot on an alien planet they should be under the laws of that planet. It's not like they were even invited. They simply showed up, beamed down and assumed that things worked the same there as they did in the Federation. The Edo are not human, regardless of appearances. They have their own culture and have the right to employ whatever laws they see fit on their world. The Edo were healthy, happy and apparently quite prosperous. Their system worked for them. By landing on their planet, Starfleet put themselves under the jurisdiction of the Edo. A first contact team is not an embassy. They do not enjoy diplomatic immunity.
And yet being tolerant of other cultures should also carry obligations on the part of the Edo -- among them, explaining possible consequences for lawbreaking and not being rigid and inflexible when cultural misunderstandings like the Wesley Crusher incident develop. The Edo were being, frankly, ethnocentric by not allowing for cultural misunderstandings and insisting on their own legal absolutism to apply to people who had not been forward of their laws and did not have the opportunity to negotiate legal agreements about the applicability of those laws towards them.
That's why it wasn't cultural imperialism for Picard to remove Wesley from Edo custody. It would
have been such if he had removed Wesley to the Enterprise
and then continued to allow Starfleet officers to stay on Rubicon III, and if he had used the power of his ship to enable those officers to do whatever they want and force their will upon the Edo. But he didn't. He took Wesley back aboard the Enterprise
, and then he left Rubicon III. The Edo continue to have their own laws, and the Federation is no longer on their turf. That's fair.