I think the puja in question is the "Grand Prayer that Blesses and Benefits all Sentient Beings", or “ in Chinese (literally means Puja of Land and Water). Its tradition dates back to Liang dynasty, when it was commissioned by Emperor Liang"

See
http://media.kmspks.org/uncategorized/water-and-land-great-festival

and in Chinese
http://baike.baidu.com/view/49829.htm

The puja is a very large scale one. The whole ceremony lasts 7 days and 7 nights of chanting of various important texts, and is divided into 7 altars or shrines. There is an inner shrine and 6 outer shrines, and they require 59 Bhikkus to lead the chanting in all 7 shrines. Other helpers are needed to help out with reception, cooking, admin, etc, and close to 100 people in total are required to organize such a large scale puja. Furthermore for the puja, for all 7 shrines, there needs to be many offerings such as oil lamps, flowers, fruits, vegetarian food, etc, and need to be replenished everyday for 7 days.

The cost of organizing such a grand puja is huge, considering the costs of tentage, the large amount of offerings required, and the amount of manpower involved. Typically (and historically), the cost of all 7 shrines are borne by a few sponsors from the laity for benefit
of everyone who wishes to participate in the puja.

The reason why the inner shrine is restricted to few people is to minimize interruptions from having too many people moving in and out of the shrine, as the chief master presiding over it has to perform complex visualizations during the puja and requires full concentration. That is why only few people are allowed in the inner platform, and for expediency reasons only sponsors are allowed.

The outer shrines do not have such a restriction and people are free to attend them to dedicate merits to the deceased or to their families and friends. The outer shrines are free because of the donations of the sponsors. There is no difference in merits accrued from the outer or inner shrines.

Of course, one way to avoid misunderstanding is to disallow anyone into the inner shrine other than the presiding monks. But that also defeats the purpose, as most people (sad to say) would not want to sponsor the puja if there is no such privilege. So out of expedient
means, and to be able organize the puja to benefit all sentient beings whether they have the means to sponsor or not, the tradition is to allow just the few sponsors into the inner shrine. This achieves two goals: to minimize the disruptions in the inner shrine as explained earlier, and to be able to conduct such a large scale puja to benefit all sentient beings.

Btw, I am not affiliated to KMSPKS, but just wanted to provide some background information so that everyone can understand the context better.