Nice try.
The real reason is that the border patrol is under orders to NOT make the Minutemen look effective. They've done it before.
See, if it were shown that civillian patrols resulted in a lot more illegal alien aprehnsions, this would go against the official fed policy.
It's politics.
I do not disagree and I would offer avoiding an international incident is a part of politics. That being said our two examples are not all that far apart, its just one looks better than the other while both have their purpose.
I really do not want to see a group of minutemen killed. Nor do I want to see what happens in this country if that does happen.