1) "THEY GAVE YOU WHAT IS ALREADY YOURS..."
QUESTION: The Mishnah at the end of Maseches Eruvin records a cryptic statement in the name of Rebbi Shimon: "When the Chachamim permitted something, they merely gave you what was already yours (i.e., what was permitted by the Torah), for they permitted only what was prohibited by a Rabbinic injunction."
The Gemara explains that Rebbi Shimon's statement is addressed to the Tana of the Mishnah (102b) who permits one to tie a torn harp-string in the Beis ha'Mikdash on Shabbos. Rebbi Shimon argues that one is not allowed to tie a string, even in preparation for the Avodah in the Beis ha'Mikdash, since a Melachah d'Oraisa is involved.
Rebbi Shimon's words should have been included in the Mishnah earlier (102b) that discusses one who ties a harp-string. Why does the Mishnah wait until the end of the Maseches to record his statement?
(a) TOSFOS (DH Amar) explains that the Tana of the Mishnah first wanted to list all of the actions, without interruption, that are permitted in the Beis ha'Mikdash and prohibited outside of the Beis ha'Mikdash. Afterwards, the Tana returned to the subject of the harp-string and recorded Rebbi Shimon's dissenting opinion.
(b) The MAHARSHA suggests another approach. Rebbi Shimon does not mean to address the specific Halachah of tying a harp-string. Rather, he makes a general statement that applies to many of the laws of Eruvin.
Throughout Maseches Eruvin, the Rabanan were lenient with regard to the laws of Eruvin and Reshuyos (for example, an army camp is exempt from certain types of Eruvin, 17b; Pasei Bira'os permit the use of a well in a Reshus ha'Rabim for travelers on their way to Yerushalayim for the festival, ibid.; Mechitzos of horizontal and vertical ropes are acceptable partitions during travel, 16b). Why were the Rabanan lenient with regard to these Halachos?
Rebbi Shimon explains that they were lenient because the Eruvin and Mechitzos in these cases satisfy the Torah's regulations. It was the Rabanan who added extra requirements. Since the Rabanan created the additional rules, they have the authority to waive them when circumstances warrant.
When the Gemara explains that Rebbi Shimon addresses the specific case of tying a harp-string, this is only because the Gemara understands from his words that he also has a specific case in mind. The Gemara knows, however, that Rebbi Shimon's statement is a general one, and that he means to encompass the entire Maseches in a broader sense. This is why his words make an appropriate ending for Maseches Eruvin.
HADRAN ALACH MASECHES ERUVIN