Have I got this right?

If anyone reads this, I'd be glad to hear if you can tell  me something I don't know. The issue is, refugees. In Australia, as in the United States, we find people trying to get into the country hoping for a better life (or on the run from something?) and claiming in some cases to be refugees. There is an ongoing argument about how we should respond, should we let them all in or turn back those who can't clearly show refugee status. It's happening in Southern Europe, too, people attempting to get to Italy from Africa by boat - sometimes with tragic results, like mass drowning when a boat sinks, or deaths in the desert when they did not manage to make one part of the journey safely. There are those who say we would solve the problem if we just opened our borders and made it easier for them to get here. Now as a Christian, I would never want to be found lacking proper compassion for the desperate. But there is good evidence that some of the would-be migrants are not refugees, they are not forced to flee from something, they simply want to get somewhere they hope life will be better. But should we just open our borders? I'm wondering: if the U.S. let everyone in who wanted to come, half the population of South America would cross the border, the U.S. population would double in five years and the place would become exactly what the migrants are trying to escape from. It would be critically overcrowded, the infrastructure would not cope, people would be living in desperation without adequate medical services, employment or drinking water, and order would break down with clashes between ethnic groups or gangs preying on the desperate.  In Australia, likewise,  if we simply said "Come who may" the inrush from places like Sri Lanka, Vietnam, China et al would overwhelm the country and cause huge problems with infrastructure, maintaining living standards and stopping violent clashes over scarce resources. So despite the moral blackmail directed at those who do not believe in unrestricted migration, it is pure necessity to keep it in check. If I'm wrong, someone feel free to tell me HOW, not just insist THAT, which proves nothing at all.
Just recently with a change of government the number of illegal arrivals by sea has dropped right off. If the respectable press is to be believed, some people from Iran and Sri Lanka who travelled to Indonesia trying to get to Australia by boat have agreed to return to their homelands. So they cannot be as desperate as they claimed. That would indicate that spurious claims to be refugees were made by some who were just economic migrants, trying to jump the queue and not go through legal channels.   
The movement of refugees around the world is inherently dangerous, and the things that cause them to be refugees need to be counteracted. Simply offering them places in another country does not stop others being made refugees. Indeed it can give rise to an evil industry in people smuggling, which has led to the migrants being tricked into getting onto unsafe boats and sent to sea where they sink. If the United Nations was any good, could it not stop the upheavals and pressures that drive people from their homes?
If you can see that I'm wrong here, show me how. It seems to me that some deceitful moral blackmail and general humbug has been generated around this issue,  by those claiming the moral high ground advocating for 'refugees' and not thinking clearly about the facts of it all.