I found this on a website ( http://www.marine-electronics.net/techarticle/battery_faq/b_faq.htm )

it says under MYTHS about batteries:
"10.6. Test the alternator by disconnecting the battery with the engine running.

A battery as like a voltage stabilizer or filter to the pulsating DC produced by the charging system. Disconnecting a battery while the engine is running can destroy sensitive electronic components, for example, emission computer, audio system, cell phone, alarm system, etc., or even the charging system itself. These damages can occur because the voltage can rise to 40 volts or more. In the 1970s, removing a battery terminal was an accepted practice to test charging systems of that era. That is not the case today. Just say NO if anyone suggests this."

So like I said, old vs new. The alternator puts out a rectified DC signal. That means it was AC but the diodes rectified into DC. there is still a small amount of ripple on the output. with the battery in place, the battery acts like a shock absorber which mellows the ripple out. This ripple can ruin electronics in NEW vehicles. Old stuff like ours aren't affected because the technology is more resistant to the ripple.