The Mishnah states that Terumah Tehorah may be moved on Shabbos, but
Terumah Temei'ah is Muktzah.
Why is it Muktzah? After all, it has a use -- it is fit to be fed to dogs,
as the Gemara in Pesachim (32a) states that one may feed Terumah Temei'ah
which is Chametz to a dog. RASHI explains that on Yom Tov, a person may not
give Terumah Temei'ah to a dog because this is considered a fulfillment of
the Mitzvah to destroy Terumah Temei'ah.
Why, though, is destroying Terumah Temei'ah not permitted on Yom Tov? The
Gemara (24b) says that it is forbidden to burn Kodshim Temei'im on Yom Tov.
The Mitzvah of burning Kodshim does not override the Isur of kindling a
fire (which is not made for the sake of preparing food). However, that
applies only when there is an Isur d'Oraisa that would be transgressed by
burning the Kodshim (such as kindling a fire). What Isur d'Oraisa is there
in feeding something to a dog on Shabbos? Why should it be forbidden to
give a dog Terumah Temei'ah?
ANSWERS:
(a) RASHI in Beitzah (27b) explains that the fact that it is a Mitzvah to
burn Kodshim makes the act of burning into a "significant act," which is
considered a Melachah on Yom Tov ("Ach'shevei l'Melachah"). Since the Torah
gave this act a high level of significance, it is included in the category
of "Melachah" and in the prohibition of not performing Melachah on Yom Tov.
(b) TOSFOS in Beitzah disagrees with Rashi. The reason one may not give
Terumah Temei'ah to his dog on Yom Tov is because there is a specific
Mitzvah to *burn* Terumah Temei'ah. (Even though the verse does not specify
that the Terumah must be burned, it does mention burning with regard to
Kodshim, and Terumah is learned from Kodshim.) Therefore, Terumah Temei'ah
is Muktzah because it may *not* even be fed to a dog, but it must be
burned. (Only Terumah Teme'ah which which is Chametz, and must be
destroyed, may be fed to a dog.)