It's not the player's place to argue rules interpretations with the DM like a lawyer making a case to a judge or jury. The 5e DM's role is to interpret (and change and add to and overrule and outright ignore and change his mind about later) the rules as he sees fit, it's the "DM Empowerment" feature of 5e.
I've played in an RPG where the players knew no rules and the DM, [MENTION=813]jmucchiello[/MENTION] , adjudicated everything. It was a lot of fun, but there was trust and consistency from him.
In an RPG with written rules, that's a common foundation for everyone playing the game, with the DM both as arbiter and custodian of the rules. If there is ambiguity, like in this case, it's up to the DM to decided how to run it. And if there is no ambiguity but the DM wants to run it otherwise, they can do that as well as a house rule.
However, it's the shared understanding of the rules that allows things to flow smoothly. If different people in the group interpret a rule differently, there should be a discussion. Yes, the DM has final say, but I've made mistakes running before and had the correct rule pointed out to me. And there have been ambiguous things (like this spell) that players have convinced me are either fine or cooler run a way different than my default would have been.
Also, if the DM runs a rule one way in one circumstance and another way at a different time, the players need to bring that up as well. "But you said two weeks ago this worked one way when that wizard cast it on us, that's why I paid to copy the spell. Why does it work differently now?" (Or worse, when there may be bias for/against a player and the rules are applied to them differently than to others.) That is in the realm of players to bring up and require a set ruling.
Not saying a DM can't change how they handle something, but it should be like the OP - the DM comes out and says "I'm going to be doing X differently because of Y and Z". Because at the end of the day RPGs are a social game and everyone has a say.