Cars have their batteries wired in parallel to the electrical system, not in series with it. Most cars will continue to run even with the battery disconnected. (Some engine electronics will detect a disconnected battery and shut down, but that's due to their programming, not to how the battery is wired.)
Despite what someone else has said, disconnecting the battery does not cause the mean voltage in the system to increase and you are not in any way "sending 14 volts through electronics designed for 12". Sensitive electronics are all protected by voltage regulators that will maintain proper voltage levels (interesting fact: many parts of a car's electrical system run at less than 12 volts, relying upon voltage regulators to drop the voltage).
Also, a car's electrical system is not designed for 12 volts nominal. Most automotive electrical systems are designed to operate at 13.4 volts nominal. In fact the lumen output specs for headlight bulbs stipulate the design voltage as 13.4 volts as this is the nominal output voltage for the alternator in order to provide adequate float charge to the battery.
That said, removing the battery while the engine is running is not recommended for modern vehicles as the battery does play an important role in surge suppression by absorbing current spikes and if you run a vehicle without a battery for an extended period of time you put extra strain on the voltage regulators within the vehicle's electrical system. In older vehicles that used carburetors and did not have computer controls removing the battery was fine, but it's generally not recommended for modern cars.