The Gemara proves that the laws of Eruvei Chatzeros are more lenient from the Mishnah (81b) that says that one may make an Eruv Chatzeros for someone else without his knowledge, but one may not make an Eruv Techumin for someone else without his knowledge. As the Kollel recorded in the Insights (46:1), Tosfos asks what this has to do with Chumra or Kula? TOSFOS (DH v'Ein) answers that in truth both Eruvei Chatzeros and Eruvei Techumin are a slight detriment for the person, and nevertheless we allow Eruvei Chatzeros without the consent of the owner but not Eruvei Techumim. That is the Chumra of Eruvei Techumim.
But that is not what the Mishnah that the Gemara cites says about the difference between Eruvei Chatzeros and Eruvei Techumin! The Mishnah says that the reason for the difference is because Zakin l'Adam she'Lo b'Fanav v'Ein Chavin Lo she'Lo b'Fanav, and that is not a Chumra or Kula (as Tosfos asked). How does the answer of Tosfos conform to the Mishnah?
S. Siegel
Tosfos hints at the answer to your question, and the RITVA here spells it out more clearly. The Gemara understood that Rebbi Yehudah's reason in the Mishnah is not the real reason for the difference, since even when we know that the person would want an Eruv Techumim on one side we still cannot make it for him without his knowledge. Vice versa, even when we know that the person does not want an Eruv Chatzeiros we can make it against his will (as Tosfos brings). The real reason for the difference, then, is because the Rabanan were more Meikel with regard to Eruvei Chatzeiros.
(The reason for the difference given by Rebbi Yehudah in the Mishnah is a secondary reason, which holds true in most cases but not in all cases as mentioned above.)
M. KORNFELD