From worldoftui.com:
A smile becomes a shared sign of quality
World of TUI gives our tourism brands a clearly identifiable international look. It maintains the diversity of our tourism brands at the same time as creating a globally visible brand-world -- a shared sign of quality, of a kind never seen before in the tourism industry. We therefore selected a smile as the new logo because a smile is one of the few forms of expression that has no national or cultural boundaries. It also succinctly expresses our vision: »Putting a smile on people’s faces
.....so that explains it then.
sB