I'm not convinced that giving XP just for killing monsters is "following the rules". How do you explain all the WoTC adventures that encourage milestone leveling?
We're coming at it from several different angles, but I think it's safe to say that most DMs here would agree that awarding XP just for killing enemies - and only for killing enemies - would not be our chosen way to run a campaign.
It is when you're judging for AL games, unless the mod is written with milestones. There are several alternatives to granting XP or advancing without using XP, but defeating monsters is presented as the default. Unless the mod is written otherwise, it's the only option an AL judge has.
At a certain point if you're rewarding XP for everything why bother with XP? It's just extra overhead. Some people enjoy it, I personally don't see the point and if games are run using just the default it rewards combat and only combat.