It’s a dumb practice. It is not the policy everywhere:
“Asia and the Pacific: Special care must be taken to ensure that your well-meaning gesture is not taken as insulting. If you are unsure, it is best not to tip. If possible, observe the locals and follow their lead.
Central/South America: Many hotels and restaurants add a service charge to the bill, and an additional tip is unnecessary. If not, 10% is the general rule for restaurant service, and a dollar per bag will be appreciated.
Europe: Many hotels and restaurants add a service charge to the bill. In most cases, an additional tip is unnecessary. If no service charge is added to your bill, 10% is the general rule for restaurant service, a dollar per bag will be appreciated.
Middle East/Africa: While your tip will not be seen as insulting, it may be unnecessary. Once again, the best bet is to do as the locals do.”