The Coast Guard, under the direction of Clinton holdover Norm Mineta DOT, said that they felt they had used the "appropriate" amount of surveillance to insure the safety of the people on board ship.
From what I heard it was Key Biscayne where they landed and from the shots on TV, it looked like the northern end of the Key near the Rickenbacker causeway, not the southern end where all the homes, businesses and neighborhoods are. That causeway is the only way (without a boat) on or off Key Biscayne and putting them off at the north end is about guaranteeing that they can easily be rounded up.
If I really wanted to dump illegal in S. Flordia, that would be about the last spot I would pick. I'd do it at night, say on Miami Beach where they could easily blend in to the city, with several bridges going to the mainland, not in an area where there are no homes or businesses and only one road and a mile+ long causeway to get anywhere else.
It's foolish to rule out plain stupidity, but if someone wanted these people to all get picked up in a high profile way, they picked the perfect spot.