Probably noted moisture in the base of the vent, assumed it was from rain penetration at the termination so they made this deflector against a prevailing wind/ rain. BTW, this voids the warranty and listing on the vent system and any attached appliances. It could be as Jim offered as well but I'm going with the condensation angle.
Recommend after they replace that mess with the listed termination that they have a qualified technician perform combustion analysis to ensure the appliances are firing properly. If there is an attached Cat. I induced draft furnace recommend a post-purge timer for at least 45 seconds-4 min. after the burner cycles off. That should reduce or eliminate any condensation.

HTH,
Bob

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Keep the fire in the fireplace.