Joining BG in combat?!

Hey there,
Today I was killing a horde on my ally character that he just disappeared and joined BG.... I wanted to ask is it supposed to be like this? that is really annoying that ppl just flee like that. I checked some websites and it seems to be impossible to join BG while in combat on Blizzard and I was curious to see if it is like that or no.

It is normal and it should be like that.

i asked some ppl who have played on Blizzard and they said it is impossible. also on Wowwiki and other sites it says it's impossible to join bg while in Combat.

it's just here but a lot of people do it to avoid dieing, you can also queue arenas too.