Well, well, well!
Good comment Ian and others. Sure, the THEORY is fine, but in reality the vast majority of carpets have been vacuumed regularly and should not need to be vacuumed before cleaning.
Obviously the logistics of pre vaccing would massively impact on the price you can charge as the time difference is substantial.
If, as some claim, they vacuum north/south, east/west and have to move furniture to vacuum then again to extract, just how much time is being added to each job and is it REALLY adding value, or just allowing you to load your price because it's taking twice as long as necessary.
Those who endorse the practice will want to show the dry soil they remove, or tell about the damage to motors / pumps by failing to vacuum.
I've never had such problems, but there seems to be a lot of posts about pumps and motors having short lives in the past couple of years.
Carpet cleaners are hired as they do something the house owner can't do themselves ! They CAN however, vacuum their carpets and might be unhappy to be charged for that part of the service, especially if they calculate the time taken in relation to the price charged.