Really??? My boyfriend says he never heard this before? That sounds unusual!
Customs change quickly, so the following explanation might be a bit outdated:
Poles are polite in the department of giving tips or buying somebody a drink or a meal. They will rarely say "I'd like to buy you a lunch", but rather "I'd like to invite you for a lunch". The invitee knows that s/he will get a free lunch, but the ugly word of "money" or "buying" is never mentioned. However the expression "Would you like to go with me for a lunch" implies that the cost of lunch will be either shared or the matter will be decided later.
Similarly, "Here is your tip, darling" would be considered impolite, degrading. After all, by definition, all Polish waitresses are little princesses. Instead the "Thank you" phrase is used.