One of the many answers given regarding Rebbi planting on Purim was that, in fact, it was the minhag of Rebbi's town to abstain from Melacha on Yom Tov. Rebbi was nevertheless able to plant because it was a Netiyah Shel simcha. If it was asur to do melacha in his town why should the fact that it was a netiyah shel simcha make a difference?
Ari, USA
The Me'iri explains that only work that involves bother and pain was forbidden on Purim. Thus a building for Simchah is exempt from the prohibition. The Ritva does not seem to assume the Heter of Binyan Shel Simchah is as sweeping as it is according to the Meiri. He says that since Purim is a day of rejoicing, Chazal did not prohibit work that is for a Simchah. There would appear to be a difference of opinion between the Me'iri and the Ritva with regards to work that is not for Simchah but at the same time does not involve bother and pain.
Dov Freedman