Why are liberals complaining about Florida Governor Ron DeSantis sending a plane full of i*****l i*******ts to sanctuary city Martha's Vineyard?
Well, there are a few reasons.

They weren’t “i*****l i*******ts”. They had applied for asylum according to our laws and therefore are in the country legally until their asylum hearing.
“Martha’s Vineyard” isn’t a city. It’s an island with a couple of towns on it. They may or may not be “sanctuary towns” — I haven’t looked — but that’s irrelevant to the problem.
Those two are enough to verify that you don’t actually understand anything about the situation and are just repeating something you’ve been told by someone in the business of provoking outrage. So here’s the real complaint:

The problem isn’t the destination, it’s the fact that he treated these people as things for political purposes that’s the core problem here. That includes the fact that they were told they were going to Boston, where there are more social services and a Nicaraguan community, and that there would be expedited job permits at the end of the trip.
If there is a hell, there’s a special place in it for those who treat people as things.

And note also that the people on the Vineyard took in the migrants and gave them what they needed in the short term — meals and a place to stay — until they could work with the state government (headed, I will remind you, by Republican governor Charlie Baker) to make arrangements to get them more help and a better situation (with, among other things, better access to ICE offices where they need to check in to maintain the status of their asylum applications).