The civlopedia suggests they were not meant to fly at high altitude during the day, which would also significantly weaken them for daytime interception missions. While I think that getting veterans is an issue, it doesn't solve the lack of survivability for planes on their first mission. That's possible, and I would agree that if making an attack grants veteran status at night, it should also do so during the day. I'm not convinced that is the correct solution, but I don't really know why, beyond the fact that the bombers can get chewed up before they get to drop their bombs. Getting rid of veteran munitions would solve the survivability problem. It is a bit of an issue for the Germans as well, making it difficult, for example, to escort Stukas. However, that does weaken the value of veterans significantly, so I'm not sure.