That's just like having to run from a UB early because of his poison spit. Or not relying on creep upgrades vs. a Fire TB. Regulus has a skill that you actually have to adjust to. Not everything is going to work against every DG. They all need strengths and weaknesses, and Regulus has very obvious weaknesses.
Regulus' Snipe, like many other abilities is simply something you have to adjust your strategy for. Nothing really imbalanced here, just a need to adjust playstyle according to your opponent's DG choice.
The problem is that, due to the long range of snipe, there's not a feasible way to adjust for the snipe.
Let's assume you're just facing one Reggie. You're in the left lane creeping and being hit for 700 every 15 seconds, 47 damage per second. 3000 damage every minute. That's a significant disadvantage that will cost you the win if not addressed.
How do you address it? You can run back to your crystal every 60 seconds to heal. You can invest 500GP every minute in health potions. You can dedicate your Heart of Life to countering the snipe damage. All of these options require a significant investment in GP that Reggie doesn't need to match.
Chasing him is out of the question because he's always at his towers.
I personally think it's a little OP but not so much that it bothers me. What REALLY bothers me is that I'm not interacting with this Reggie. I'm not playing a multiplayer game at this point. It's not any fun. If there was an answer that allowed me to interact with Reggie I'd be OK. But sitting there being a pin cushion that can't return the damage isn't enjoyable. This is still a game and having fun is as important as winning.
Snipe is meant to finish off a weak DG or to aid a teammate in fighting a DG. If the damage snipe does is healed in 3 seconds, both of these goals are still attainable. Stupid DGs who don't run early enough still die. Your teammates still have the support of your snipe, provided they can finish the baddie off. It's a good solution that doesn't comprimise the core purpose of snipe.