Halo's maps were all incredibly small maps outside of a handful like Infinity, Sandtrap, Avalanche and Sandbox. So the sniper is going to be used in a lot more close quarters than it would normally be, which is fine.
Another counter to this is the fact that the Sniper in halo games outside of specific game modes like Shotty Snipers, is a power weapon that only spawns on the map after a set time limit, it's not something you can purchase and spawn into the game with.
Again, the sniper itself isn't the problem, it's the fact that a thermal scope allows you to see across the entire map and kill people off spawn that is. The Thermal Scope needs a cut off range and it'd be fine.