The contract is attached to the iPhone, and Apple doesn't want the contract sold for less than $X. If the carrier doesn't want to do that, they can choose not to sell the phone. What's the issue?
https://en.wikipedia.org/wiki/Price_fixing
"In neo-classical economics, price fixing is inefficient. The anti-competitive agreement by producers to fix prices above the market price transfers some of the consumer surplus to those producers and also results in a deadweight loss."
It also send invalid signals into the economy about the value of an item that may create bubbles or unreasonable expectations for low prices that are unsustainable (and therefore anti-competitive).
The Diamond trade is a very good example of price fixing where otherwise plentiful and cheap stones are controlled on the supply-side to keep prices artificially high.
At the same time, it is likely to be anti-competitive to allow an upstream manufacturer to fix prices with all retailers on the basis that customers are prejudiced and retailers are unable to compete on price. This is the main answer to your question. Also if only one retailer plays ball with Apple then they have a monopoly position again to the likely detriment of customers.
The arrangement is however acceptable where the retailer plays the role of mere agent in the arrangement (i.e. takes minimal risk in the transaction as compared to a distributor) as here it is reasonable for the manufacturer to set downstream prices.
However here if the article is correct, then the presence of a clear right for the distributor to set prices which is then subsequently ignored by Apple would seem to justify action by Taiwanese FTC. If anything this would be a breach of contract issue also providing another ground for the FTC to intervene.
Perhaps in this case, Apple intended to make the contract transparent by granting the right to the retailer to determine resale prices (in compliance with local law potentially) but then behind the scenes exerted pressure. This is pure speculation of course.
This phrasing betrays a complete ignorance of how the mobile market works outside of the US. Most markets have decided that a device and the network connection are separate entities and a vast majority of the world's population buys their "device" and "sim card" separately.
Hence a device manufacturer trying to set the price of a related but distinct product (the network connectivity) is something like -- to pick a favorite American analogy -- Ford not only dictating what brand of gasoline you'd put in the car, but also dictating the price you'd pay for that gas.
That is incorrect.
It is similar to the US in the UK, but not at all elsewhere. There are some contract deals in EU that go along with a carrier locked phone (like in the US), but most are not. The unlocked phone is the mainstream case.
Here is a relevant article with the different practices: http://en.wikipedia.org/wiki/SIM_lock
That you bought a carrier-locked phone in France is not really relevant, as I didn't say there are not carrier-locked phones in Europe, just that the carrier system is not as it is in the US, and having unlocked phones, pre-paid cards, and even mandatory unlocking is (and has been) the norm since mid-nineties.
In France, for example, the law (in effect since 1998) states clearly that even if they sell it locked, the carrier must unlock your phone after 6 months. That's the reason the iPhone had to be sold unlocked in France even back in 2007, when there was no such option in the US.
Heck, the US didn't even have SIM cards and GSM phones until quite recently (late '00s).
Phones sold, in France on pay-as-you-go (forfait bloqué) and monthly deals (forfait) come with a locked phone. Yes, they may be unlocked at any time (http://assistance.orange.fr/desimlocker-votre-mobile-2839.ph... and http://assistance.sfr.fr/mobile_forfait/mobile/desimlocker-m...), much like the can in the UK (http://service.o2.co.uk/IQ/SRVS/CGI-BIN/WEBCGI.EXE?New,Kb=Co...) or indeed in Germany (http://www.t-mobile.de/sim-lock-entsperren/0,20638,23789-_,0...). The end user must still pay the remainder of their contract even if they choose to use another provider. SIM only deals; either 30 day, PAYG or 12 month contracts, are commonplace. Suggesting that in anyway they are the norm is incorrect.
EDIT: Funnily enough one of my parents, a resident of France, has had an iPhone 3G, 3Gs, 4s and 5s on contract with Orange. Not one was sold unlocked with their contract.