Actually, it seems to me:
1) I saw somewhere in that long thread about residency (the one Steve started, I think, which I haven't seen in quite a bit of time), which also included a lot of information about the silliness (or danger) of the "visa run", that bajo_cero mentioned a couple of times that being here after your 90 (or extended 180-day) visa was expired wasn't illegal, but was merely "not regularized." I was sure I remembered bajo saying that it became illegal when the court issues the 30 day notice to regularize your status and the 30 days was exceeded, that at that point you are here illegally. As I understood the argument, that is why "visa runners" were not illegal, but merely not regularized, though it is within the power of immigrations officials to deny entry (which is true for even someone traveling here for the first time, technically) or initiate the immigrations court proceeding to require that person to regularize his or her status. Perhaps I misunderstood.
2) When an Argentine immigrations official lets you in the country and he or she has seen that you have been in the country for more than 180 days, and not only that but this is something that has occurred for quite some time (even unto years) - how is it exactly illegal? Is the immigrations officer committing a crime by letting a criminal enter the country? Or is it not a crime to knowingly let criminals re-enter the country when they have no regularized status? Or is it that, indeed, this is not illegal because the official is letting you enter and a court has not told you to regularize or get out?
3) If this were really a problem in general, why would Argentina not revoke visa-waiver status for people from the US, Europe, Australia and many other countries that originate perma-tourists? That would solve any "problem" for any new visitors, and any perma-tourists who left and came back.
Try entering the US without a visa (if you are lucky enough to come from a country that belongs to the visa waiver program), overstay your time and then try to get back in. Trying to do so is not illegal, but you will be promptly turned around, I believe without exception. If you are found in the country without an immigration official having let you cross the border, you are there illegally. And that would be exactly the same here, as well.
One thing I admire Argentina for is its relatively open immigration policies (even to getting citizenship when being here for years without a regularized immigration status). The only problem I see with it is the fact that a lot of poorer expats and immigrants are indeed something of a strain on the system where things are given out free to anyone who is here. But that's a problem anywhere that has social programs open to all.
I wish the US's immigrations policies were similar. Hell, I can't bring my wife with me to visit the US simply because she can't get a tourist visa. My parents are getting old, can't travel such a long distance to come visit us here, and have never met my wife. I've tried many times to get her a visa but the bureaucratic and legal idiocy (in my opinion) won't let it happen. The only way right now she can see the US is if we actually immigrate there (well, she'd be immigrating), which is ridiculous because for the foreseeable future our life is here, in south America somewhere.
It would be easy enough for Argentina to stop anyone who violated their immigration regulations if they wanted to do so. The reason expats from all over the world do it here is because it is allowed. Pure and simple.
And BTW - I know for a fact there are plenty of non-Argentine Mercosur expats who overstay their time without getting regularized, it's not just "1st worlders". These Mercosur residents don't have much of an excuse because all they have to do is apply for Argentine residency, where non-Mercosur expats have to find a program that fits them, many of which cannot find one that they meet, even though they could be just as productive as most Mercosur expats.
Again - why do people from all over the world come here and do visa runs or stay in the country beyond their 180 day per year limit? Because Argentina permits it. One doesn't need any other reason or excuse.
As to not paying taxes - that's a different story. As an AnCap libertarian, I don't believe in taxes (not just that I don't believe in them, but think it is a serious tyranny of the masses at best) yet I still pay taxes here. I didn't before I had residency because I couldn't. Until I got married, I had no program under which I could get residency. If Argentina was worried about collecting taxes from the relatively small number of perma-tourists, all they'd have to do is open up residency to anyone who wasn't a criminal and deny re-entry to anyone who kept trying to play perma-tourist.
This whole thing about "first world" perma-tourists being illegal and everyone moaning about the fact, while Argentina stills lets them enter legally, seems to be a really silly argument to me.