I have seen the seating as another "suggestion" from some unknown source. What often happens is that they will verbally issue a suggestion like this through the hounder-hounders to the congregations, or to write a vague "suggestion" such as "It might be advisable to..." in the KM inserts. What then happens is that the local congregations see these "suggestions" and decide that anyone who doesn't follow them is "spiritually weak". The effect is that they become practical requirements.
As for the seating, this means that they "suggest" that the congregations sit in the section that they are assigned to clean (hence, together). The attendants will not round up anyone who violates this, and in fact many will scatter. However, once back in the congregation, the ones who do not obey will be marked as "weak". After all, he who is unfaithful in what is least will be unfaithful in what is most as well. That usually works.
Disfellowshipped ones usually manage to find their way to the back anyway. These ones are already spiritually "dead" and have nothing to lose if they sit in the back. They will not be allowed the "privileges" of cleaning anyway, so they are usually not expected to sit with their congregation. Inactive ones might have a bit more problem here, but unless they are serious about coming back, they can usually get away with sitting pretty much where they want (as long as the section isn't roped off).
In the future, with the Kool-Aid edition of the Watchtower, they might get stricter in this area. For now, it's just a "suggestion" that will have you branded weak if you disobey. I can only hope that there will be enough people that will just blow out altogether if they can't sit where they want.