Marketing 101 says to recognize and appreciate your best customers.
It costs United NOTHING to have a flight attendant say thank you for being a 1K or GS member.
I was on a flight last week from ORD-DEN (in coach) and the flight attendant came with a business card with a thank you note from the Captain. Little things like this make you feel appreciated.
I do agree that everyone in first should be treated equally though - regardless of how you got there. But that doesn't mean they should say thank you for flying every week.