As we travel as our own family it is not a major problem for us, but on the two occasions we booked on tours they added into the timetable factory visits. As a rule these are generally just an excuse to take you to a huge store and try to get you to buy.

From my own experience and after chatting with various other people the worst offender for this is would seem to be grayline tours in Hong Kong. On our trip time at the intended destinations was reduced after wating too much time in a jewelry shop. My family and I (along with others) left the tour part way through as the guide insisted on another market visit, rather than allow people to walk up the steps to the Giant Buddha.