not sure but think the turn back situation is complicated
a bit like the issue aust. has with boat people
the mexicans turn a blind eye to a non-documented family travelling from honduras to the usa
a usa border patrol finds them just after they have crossed the reo grande
they can't really force them to swim back, and even if they did, the migrants would just wait a bit and swim to the usa again
so the patrol cuffs them and takes them to a processing centre, for; food, water, beds, baths, asylum claims
this is where the separations have started happening due to a tougher "rules based" system charging the adults with border violation......
the patrol can't really take them and dump them at a mexican border crossing as they aren't mexican and have no documents.....(remember the jungle camp in calais)
the usa could fly them back to honduras, if the migrants admitted that was where they were from, and they agreed to go voluntarily.......otherwise cuff them with border guards andflyp them where?....naru?
this is why a wall looks so appealing to trump
much the same issue in europe now
should a country be allowed to control its borders?
should a country be allowed to control its immigration?
tiny illegal migration flows have traditionally been managed in an "individually expensive" way
mass flows can not as they provoke a democratic kick-back from the bottom rank of voters who expect their gov. to spend on them, not people wanting their jobs and benefits
just look at the anti-immigrant situation here against well-behaving, well-educated, well-motivated young asians who contribute more to the tax system than they take
if we had land borders with papua new guinea? and huge illegal immigration from there
the nz public would be screaming blue murder