Not just "cheap" stuff - and Apple's power supplies are not inexpensiveCheap chargers can have poor separation between mains and elv sides ...
There tends to be some direct connection between primary and secondary - it may be "direct" by (eg) a resistor between one side of the secondary and primary to leak away any static, or it can be by way of AC filters which tend to be a pair of capacitors between the two input lines and "earth". Where there is no earth, then these capacitors form a voltage divider pulling that "earth" to around half the line voltage. So in the case of the Apple stuff, if you use an adapter/cable with an earth connection then the tingling goes away.