You might just be right, perhaps leaving a 'measured' amount of detergent in the carpet might just result in more regular work from the same people. (not that I would advocate such methods)
This was common practice ( not necessarily deliberate ) years ago when the only products available were high foaming and soapy detergents.
The difference today is huge and I've found with return customers of the once or twice a year variety, that in general, the carpets do stay cleaner for much longer but, fortunately, people get careless and tracking will always occur so they still need attention.