I'm curious. If the letter of the law stated illegal immigration was a felony would you then support citizen's arrests or have you embraced the libertarian's immigration views?
Even if the law stated that illegal immigration was a felony, you could still run into problems.
First, you would have to witness the crossing (i.e. actually see them cross the border).
Second, you would have to determine their citizenship and immigration status. They could be US citizens, resident aliens or illegals. And remember, US citizens and resident aliens are not required by law to prove anything to Joe citizen as to their citizenship or immigration status.
Third, the law states that you have 24 hours to report your crossing to immigration authorities.
Conclusion, if you don't know the law, don't try to enforce the law.
The reason Simcox and the other groups don’t get into trouble, is that they don’t make arrest or detain the suspected illegals. They just follow the group and report the location to the Border Patrol.