Eruvin 80B

שאני התם דליכא מחיצות.

The Talmud answers: It is different there, as there are no partitions. That alley is breached, and it is therefore fitting to compel its residents to establish some sort of partition, if only for the sake of protection. However, if there are partitions, one is not obligated to join in a merging of alleyways.

לישנא אחרינא, מצד שאני.

Another version of this explanation: From the side it is different, i.e., the preparation of an eiruv does not involve an adjustment to the alleyway itself, but rather it is a side issue. Consequently, an individual cannot be forced to participate in its preparation (Meir Netiv).

אתמר, רב חייא בר אשי אמר: עושין לחי אשירה. ורבי שמעון בן לקיש אמר: עושין קורה אשירה.

It is stated that the amora’im disputed this issue. Rav Ḥiyya bar Ashi said: One may build a side post from the wood of an asheira. Although it is prohibited to benefit from the tree and it must be burned, setting up a side post is a mitzva, and mitzvot were not given for benefit, i.e., the fulfillment of a mitzva is not in itself considered a benefit. And R' Shimon ben Lakish said: One may build a cross beam from the wood of an asheira.

מאן דאמר קורה ⁠— כל שכן לחי, ומאן דאמר לחי ⁠— אבל קורה לא. כתותי מכתת שיעוריה.

The Talmud clarifies their opinions: With regard to the one who said that one may build a cross beam from the wood of an asheira, R' Shimon ben Lakish, all the more so he would permit a side post to be prepared from an asheira. And the one who said that one may build a side post from an asheira, Rav Ḥiyya bar Ashi, spoke only of a side post; but as for a cross beam, no, he did not permit one to build it from an asheira. The reason is that an asheira must be burned, and it is therefore as though its size has already been crushed. Consequently, a cross beam, which must be at least a handbreadth in size, may not be prepared from an asheira. With regard to a side post, by contrast, a minimum width suffices, which means that even wood from an asheira is fit, despite the fact that it is viewed as burned.