That's been the default behaviour for as long as I can remember. I'm fairly certain of my 2008 Macbook Air doing this. Less so (but still better than even odds) for my 17" PowerBook.

You'd want some minimum charge to, at least, safely start up and immediately shut down again. Maybe throw in a little extra to be on the safe side. The higher-performance MacBooks also operate within a an extreme width of power requirements, from about 5W to close to 100W. I'm not sure if they can throttle based on battery status. But if not, you'd need a buffer to accommodate load spikes.

I guess you could criticise them for not trusting you to keep it plugged in. But any UI to clearly communicate that unplugging now might cause data loss is bound to be a horrible sludge. It could also easily fail if you happen to trip over the cable or the power went off.