I think that the WTS has designed the cart witnessing behavior to accommodate the use of them in countries where evangelical religions are strictly controlled.
The WTS has to make the rules and regulations consistent across their operations worldwide in order to give substance to the claims that they don't try to convert people at their carts.
For example, the use of carts has been the subject of court cases in Russia:
http://forum18.org/archive.php?article_id=2293
Jehovah's Witnesses E. Shevchenko and Ye. Zherebilova, for example, were charged under Article 20.2, Part 2, with "unlawful picketing" for displaying religious literature on information stands in a park in the Rostov Region town of Shakhta. Police and prosecutor's office staff carrying out an "anti-extremism inspection" allegedly observed that the two women were standing only 25 metres apart (the minimum distance permitted between one-person pickets, which require no notification, is 50 metres). Judge Lidiya Cherepanova of Shakhta City Court found them guilty on 4 April 2016 and fined them each 20,000 Roubles.
Jehovah's Witnesses Oleg Shekhanin and T. Shekhanina, however, were also charged under Article 20.2, Part 2, with "unlawful picketing" for standing together with religious literature displayed on a trolley on the river embankment in Kaliningrad – but were acquitted. Judge Irina Kuzovleva of the city's Leningrad District Court decided on 22 January 2016 that the defendants "did not take an active part in the event by expressing opinions or making particular demands, and did not create a threat to public order or public security".
The org's "rules" around how cart witnessing is to be done conform to what is necessary to evangelize in countries where it is restricted.