A quick post to support my statement in the previous post on this subject that:
It is important to note that the general rule that we are lenient regarding eruvin does not apply to the first step of determining whether an area is biblical public domain. If carrying is only prohibited on a rabbinic level, then we can say that we are consistently lenient since we are only dealing with a rabbinic prohibition. However, here we are still trying to determine whether or not we are dealing with a rabbinic prohibition, so it is premature to be consistently lenient.The Maharatz Chajes (Glosses to Eruvin 41a) explains this concept and quotes the Yad Malakhi (184), who in turn quotes the Ritva (Eruvin 89a sv. ve-ha di-snan) and Rashba (Eruvin 80b sv. aval).