The healing pools are introduced in Half-Life in a way that lets the player discover how they work. You see some oddly colored water with particles coming off of it, and an angelic, inviting tone is playing from it. Of course that gets a player’s curiosity, so they step in, and find out it heals them.
Half-Life is a series that’s at its best when it lets the players do what they want and discover things for themselves. You’re not spoon-fed information like in some modern shooters.
Take the friendly vorts in Xen for example. The only way you can know they’re friendly is if you take a moment to realize they’re not attacking you, instead of going in guns-ablaze. You’re not shown a cutscene of the vorts seeing Gordon and not attacking, and you’re not given a “Entering Neutral Territory” message or anything like that. It’s all for players to discover, and it’s better that way.
But for the sake of argument, I could totally see that limping houndeye being given the lowered gun treatment, like when Gordon looks at a scientist or guard. You can still shoot, but the gun’s not in their face, indicating friendliness.