Only the first time and you know it. Second time - and all further entries that someone is arrested and convicted for - are felonies.
Did the two guys who made the arrests know, FOR A FACT, that these two were repeat crossers when they made the arrests?
Someone witnessing an illegal border crossing has no way of knowing whether someone has previously crossed, so it doesn't rise to the threshhold of probable felony.
Don't get me wrong, I think the laws should be changed to allow armed citizen apprehension of illegals where they are witnessed crossing the border or there is a high level of certainty that they are illegals. But under current Arizona state law, being a probable illegal does not rise to the citizens arrest threshhold. ABP recognizes this, and does not attempt to apprehend illegals unless they witness some other offense, such as breaking into a home or burglarizing of an outbuilding.