Yerushalayim, Visitors, and the Public Practice of Carrying
Public carrying teaches by sight. Rav Moshe considered what visitors would understand when they saw observant Jews carrying in a city whose unusual walls or local psak they did not know.
What a visitor learns from public practice
Most visitors do not know the walls, leases, street classifications, or local psak behind an eruv. They see observant Jews carrying in the street and remember that carrying was accepted there.
When they return home, that memory can outlast the local conditions that made the original practice possible. A person may assume that a similar-looking street is also enclosed or that the same Rabbonim permit it.
Rav Moshe's discussion of Yerushalayim
In Orach Chaim I:139 §5, Rav Moshe suggests that Yerushalayim was not joined through an eruv because people came there for Yom Tov and throughout the year. They might see carrying and later carry at home where the same enclosure did not exist. He draws an analogy to Manhattan and nearby places that could not be repaired in the same way.
Orach Chaim V:28 §§13–15 returns to the point with an important limit. Rav Moshe does not apply the concern automatically to every ordinary city; he says it may be especially relevant to modern New York, which receives people from throughout the world, and perhaps to Brooklyn, assuming the enclosure has no independent defect.
Crown Heights and the wider Lubavitch world
Crown Heights receives guests, bochurim, families, shluchim, and Rabbonim from Chabad communities throughout the world. Its Shabbos practice is watched and often treated as a model.
Public carrying in Crown Heights could therefore shape assumptions far beyond the neighborhood. The local prohibition protects both the people present and those who may later carry elsewhere on the basis of what they saw.