Mike,
A lot of this comes down to just what are you asking? Are you referring to what happens during Mass, or outside of Mass? And just who are you asking about?
Liturgical laws of the Church do not say “who is not allowed” but rather say “who is allowed.” And only those who have some legitimate purpose for being there, are those who should (or may) be there. Every person at Mass has a proper location. The Church doesn’t say “the laity are forbiden to enter the Sanctuary” but instead says that there is a proper place for the laity and a proper place for the clergy (and those who have a reason to be in the Sanctuary).
GIRM 295 defines the legitimate purpose of the Sanctuary. It’s not a place for people to be seated simply for the sake of being there, or being seen to be there.
Here’s the relevant section from the GIRM
- The People of God, gathered for Mass, has a coherent and hierarchical structure, which finds its expression in the variety of ministries and the variety of actions according to the different parts of the celebration. The general ordering of the sacred building must be such that in some way it conveys the image of the gathered assembly and allows the appropriate ordering of all the participants, as well as facilitating each in the proper carrying out of his function.
The faithful and the choir should have a place that facilitates their active participation.
The priest celebrant, the deacon, and the other ministers have places in the sanctuary. Seats for concelebrants should also be prepared there. If, however, their number is great, seats should be arranged in another part of the church, but near the altar.
All these elements, even though they must express the hierarchical structure and the diversity of ministries, should nevertheless bring about a close and coherent unity that is clearly expressive of the unity of the entire holy people. Indeed, the character and beauty of the place and all its furnishings should foster devotion and show forth the holiness of the mysteries celebrated there.
- The sanctuary is the place where the altar stands, where the word of God is proclaimed, and where the priest, the deacon, and the other ministers exercise their offices. It should suitably be marked off from the body of the church either by its being somewhat elevated or by a particular structure and ornamentation. It should, however, be large enough to allow the Eucharist to be celebrated properly and easily seen.