Read the original news articles in French --- they don't say 20%.
What they said was 80% signed up with a "iphone specific" plan.
http://www.20minutes.fr/article/198973/High-Tech-30-000-iPhone-vendus-en-cinq-jours.php
It ought to be pointed out that '20 Minutes,' the free, advertiser-owned evening paper/website in/on which this story appeared, isn't too reliable and often cites weird numbers without explanation. Here, all we have is a note at the bottom that suggests some of the info came from AFP.
In any case, your observation about the falseness of 100-80=20 in this case is well-noted.
But the law itself doesn't achieve that at all. A french carrier can still either lock in their customers with an abusive contract (the french iphone tariff is the worst among UK, France and Germany) or as the iphone illustrates, the carrier can charge an abusive simfree handset price.
I'm not saying you're wrong--I don't really know--but it is worth pointing out that, in some cases, French stuff tends to be more expensive because doing business here tends to be expensive; I know from personal experience how hard it can be. French labor laws are strong, unions function in a different way from American unions and are unbelievably powerful, and it is very, very difficult to run a business at a profit...and if you do pull it off and do too well you get killed by taxes. 35 hour workweeks, 5 weeks minimum paid annual vacation and generous unemployment benefits are great, but we pay the price in many ways, and one of them is expensive goods and services, all in a market where salary growth has been virtually zero for years. Do companies hide behind this to inflate prices? Damn straight they do...but it takes a lot of research to figure out who's doing it and who's just trying to make an honest living. And then, of course, the government tries to protect the markets with price-fixing laws to stop companies from gouging in the name of high business costs...and it gets really damn complicated.
I disagree. Those European laws haven't done a single thing at all.
Europeans are getting so-called cheap phones because you got carriers facing lax public accounting laws which permit them to chase "fake" subscribers --- it's just churn. Also Europeans are getting cheap phones because VAT fraud greases the whole handset retail industry. The cheapness of the cell phone handsets have nothing to do with these European simlocking laws in question.
In particular, to have all these French simlocking laws but only allows 3 national carriers --- defeats the whole point for consumer rights and lower prices. That's penny wise and dollar stupid.
I want some links here too...again, I don't know any better, but you can't say all that and then give a link to an article that doesn't say one word about the French market

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Regarding the three 'national carriers': it ain't that simple. If you can figure out the goings-on vis-a-vis portable phones of the "Autorité de régulation des communications électroniques et des postes," please explain it to me because I don't get it. What I get so far: The physical aspect of the network, which is (I think) entirely owned by the state, are shared by all three big carriers (including the costs of maintenance) and create a network (which covers virtually all of France), which is then leased to about 30 other companies who have the legal right to buy access to it. This number grows regularly, as even grocery-store chains are in the game now. To further cloud the picture, the company that we're all talking about, Orange, is officially listed as a 'société anonyme,' which is a type of private company...but it was 'purchased' by France Telecom, a public company currently in the throes of some strange half-ass type of privatization, in 2000. I don't know what this means. In the end, I have no idea what the France Telecom portion of my taxes pays for. And then there was the whole deal with British Telecom...argh.
As confusing as this all is, I haven't seen anything suggesting that the French government has actually prevented anyone from entering the market at any level. It is very possible that the failure of Bouygues to compete with SFR and Orange in terms of market share, despite 11 years and counting of major efforts, has scared away other would-be multi-billion dollar investors...but that's just a guess.