Could it be this simple?

An invisible wall exists around every nation, erected by international laws to protect the assets and interests of each bordering country.

Provisions for passage exist with conditions to be met and respected. Continued breach and disregard of that agreement will inevitably require a reinforcement of the invisible wall with something visible. Not due to a fault of the host country but due to the blatant disregard of the infiltrating elements and leaders who turn blind eyes to it.

The fact that a once invisible wall must become a barrier was never the the original intent, but is made so and considered such only by its violators.

Appealing to the hardships, heat and distress suffered by those who try to cross not seeking actual asylum and against the law in places where no walls exist is invalid. Knowing there exist walls or fences in some places–and for good reason–then actively seeking passage in places they don’t exist, is having knowledge that it’s wrong and prohibited but not caring. Ignoring the very first law encountered in what they hope to be their new home questions whether they will respect any others if asylum is granted.

And still, the signs don’t say ‘keep out.’

They merely say ‘please use the door.’