How can they do that?

Anything lower than 50volts is considered "safe" by the electronic industry and does not necessarily require special safety features. Also, A/C is considered more dangerous than DC.

You will note that many cars have exposed 12V terminals on the battery and you can grab both terminals without being "bitten" :-)

Another good example is the telephone circuits in a home may have up to 35V A/C but are considered "safe" and do not require safety protection.

JZ

This is wrong: SELV - as defined by the safety agencies - is defined as 42 V AC rms or 60 V DC (~42 * sqrt 2). When you shorted the conductors, you were saved by the fuse, put there intentionally to protect the user and equipment from over-heating.

When a reputable car manufacturer designs a car, they will take into consideration the chances of someone actually bridging the battery posts. A good design will include post covers to prevent accidental short (that can be a serious thing).

On the telephone side (PSTN): the voltage between Tip & Ring when in on-hook position is -48 V DC nominal (this is dependent on your distance from the CO or RT, and what wire gauge was laid). It is negative with reference to ground. This voltage has a current limitation of 30-60 mA. On ringing, you have ~90 V AC (20 Hz) super-imposed on the -48 V DC, so there is a chance you will see up to 130 V momentarily.