Consumerist has a sad story about a woman who was electrocuted by a cheap USB charger. The danger seems greater in countries that use 230 volts around that house rather than 115 like the United States, but even 115 volts can be dangerous if it crosses a vital organ.
A charger’s job is to take the higher AC voltage that comes from the wall and convert it to 5 volts DC to power the phone or tablet plugged into it. Poor design or poor manufacturing can cause the wall voltage to go where the converted voltage is supposed to. Literally, getting the wires crossed in this situation is very dangerous.
The solution is to be careful where you’re buying your chargers. Don’t buy them out of the back of a van, and don’t buy them from the dollar store. Get a charger made by a company you’ve heard of, and look for a regulatory logo on it. Approved products sold in the United States generally will bear a UL (Underwriter Laboratories) logo on them, while approved produces for the European Union will bear a CE (Conformité Européenne–European Conformity) logo.
I agree that many chargers are greatly overpriced, but there are reasonably priced, safe third-party chargers available too, such as the Amazon Basics USB charger, which costs around $9. Some stores have their own house brand that are similarly priced. Even at $9, there’s a comfortable profit margin for the retailer.