Ken Shirriff took apart an Apple iPhone charger and found quality in a tiny expensive package:

Apple’s power adapter is clearly a high-quality power supply designed to produce carefully filtered power. Apple has obviously gone to extra effort to reduce EMI interference, probably to keep the charger from interfering with the touchscreen. When I opened the charger up, I expected to find a standard design, but I’ve compared the charger to the Samsung charger and several other high-quality industry designs, and Apple goes beyond these designs in several ways.

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Apple’s design provides extra safety in a few ways that were discussed earlier: the super-strong AC prongs, and the complex over-temperature / over-voltage shutdown circuit. Apple’s isolation distance between primary and secondary appears to go beyond the regulations.

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Apple’s iPhone charger crams a lot of technology into a small space. Apple went to extra effort to provide higher quality and safety than other name-brand chargers, but this quality comes at a high cost.

This does explain why the iPhone touchscreen may fail to respond correctly when plugged into 3rd-party USB chargers. So, for safety’s sake, please don’t get cheap knockoff USB chargers, at least get decently priced 3rd-party chargers or Apple’s.