When selecting a light preset it changes the lights for the scene you're working in, including altering the lights (and amount of them) that are there.
For instance, if you start with a basic Infinite (default) light then switch to the Fire preset, you'll notice 2 lights now, and the colors and settings of both lights has been adjusted to create the Fire preset.
So it does change the current light's color, as well as adding a new second light with a color to get the desired effect intended by the preset.
Moving from there, Select the Hard Lights preset --
Now you have 3 lights, all of them different color from the Fire colors (which were reds and orange, and now all 3 are grays)
Is this not what you're seeing Howard?
Yep, this is what I'm experiencing, though it doesn't seem like that should be the behavior. Usually, when selecting a custom preset with an active layer/element, it changes the properties of that layer/element.
For instance, If I add a Curves Adjustment Layer, and choose a custom preset, it doesn't add a new Curves Adjustment Layer. It simply updates the current selected one.
To the average user, if they wanted to add a new light, they would use the New Light button, then select a preset, if desired.
If this behavior is 'as designed', the label should read something like "New Light from Preset". Nothing tells the average user that selecting a preset will add a new light.