Because he's not asking the rules about surprise. And it may not be RAW, but the way it was played is not a huge leap. It's reasonable for a DM to allow a spell to be cast outside of combat to start the combat. It's just like playing the first round of combat where the enemies are surprised and the other PCs don't act.
That's a pretty powerful upgrade. And takes out a lot of the risk casting a second concentration spell. Say you cast hold person while concentrating on wall of force and the enemy makes their save. Then you could say that you're still maintaining wall of force and you haven't lost anything (other than the spell slot). Even if you say you have to decided before the save is rolled, and you picked hold person which was then saved, wall of force would still be up for a round, giving you control of the battlefield for the next round until you could recover from it.
Either way, you're increasing the power of spellcasters and reducing the risks associated with casting concentration spells while already concentrating.
I don't see a reason for this change, other than wanting to have your cake and eat it too.