Because the behavior is merely described, not enforced. It's relying on everyone to obey the two design rules that: 1) only the writing side should close the channel and 2) only one side should write.
In this sense, it's as safe as mutexes: everything works as long as everyone (including any 3rd party library) does exactly what they're supposed to. When compared to what modern and robust concurrent programming looks like (e..g Erlang), it seems like a compromise to me
