Why does a mechitza not require bittul:just the fact that something is there is enough.
Whereas by Charitz, to be a valid Stima it has to have
Bittul.The fact that it is flled in is not enough.
Philip Weitz, Toronto Canada
The Charitz requires bitul because this is a ditch which was filled with straw or stubble. The normal place for straw or stubble to be found is not a ditch so if he does not do bitul there is good reason to believe that he will remove it later on from the ditch. In contrast we have no reason to think that the owner might remove the Mechitzah, so bitul is unnecessary.
I should just add to the above that the reason we are not concerned that the Mechitzah might be taken away, is because people are aware that the Mechitzah gives a heter to carry inside the chatzer and they will not make the mistake of saying that they can carry in the chatzerot even if the mechitzah is removed. Chatzitzah in the ditch is different, because people do not so readily think that the reason they were allowed to carry is because of the earth filling the ditch. Therefoae, if one did not do bitul on the earth in the ditch, and then the earth was removed from the ditch, one may not be aware that the reason that there was a heter to carry was because of the earth.
People understand that a Mechitzah gives a heter to carry and that if it is removed one no longer possesses the heter, but they do not appreciate this point in connection with the earth.
I found, bs'd, an answer to this question in Moreshet Moshe, by Rav Moshe Steinhouse, Eiruvin page 536, #5.
1) He explains that there is a significant difference between the Mechitzah and the Charitz. The job of a Mechitzah is to prevent the "Regel Harabim" getting through. A Mechitzah stops people walking through. But a Mechitzah does not need to become a part of the place where it is situated. That is why Rashi writes that it does not matter whether the Mechitzah will in future be removed or not. The crucial thing is that the Mechitzah is there at the moment, and is stopping people from getiing through from one chatzer to the other. If the Mechitzah is effective at doing that at the present moment, this means that the 2 chatzerot are separate, and each one requires its own eiruv.
2) The Charitz is different. When the ditch is filled up with straw, we are trying to nullify the ditch. If we can do that, this will mean that there is no longer any division between the 2 chatzerot, and they will only need one eiruv. However to be able to do this, it is essential that the straw remains where it is now. If it does not stay here, then the ditch comes back to life as a separation. That is why we must have bitul; to ensure that the straw becomes a part of the former ditch where it is situated.
3) This is the difference between the Mechitzah and the Chatzitzah. The Mechitzah does not need to become part of the place where it is now. Therefore bitul is not required. The Charitz is different. The Charitz must be considered to be filled up. The straw must stay here. Therefore for the straw to be a valid Stimah, bitul is necessary.