It's perfectly understandable that OP asked the question as tipping isn't the custom everywhere. But the curbside check-in folks in the US are not employees and make their money from tips. It's unfortunate that they may feel the need to remind people, but it's likely their experience that people from other countries don't know that tipping is expected.

Do bear in mind that unlike other services, you really do have a choice. Curbside is a major convenience for many people. But, you are free to lug your stuff inside to a counter, check it there with an air carrier employee and neither is a tip expected nor would one (hopefully) be expected.