Not an electrical engineer nor an expert on this matter, but there used to be a time where cell phone bands weren't well regulated and the EM waves would interfere with communication devices. I remember this being a much bigger problem when I was in high school, where even cell phones on silent would interfere with the wireless microphones for our stage productions.

But I do know the reason they ask for all devices to be put on airplane mode or turned off - it is because it's easier to issue a blanket statement than require specific devices to be turned off.

I'm assuming the EU evaluated that the risk is nearly negligible - probably because the odds of anyone using such outdated tech is practically null.