Kol Koreh Against Heter Mechira

Yeshiva World is reporting that Rav Eliyashiv (and others) has signed a Kol Koreh against the use of a Heter Mechira (temporarily selling land in Israel to a non-Jew) in the coming Shvi'is year. As per YW:

The Kol Koreh states that the Heter Mechira leads to the desecration of the “holy and precious” Mitzvah of Shmita.

Now, I'll be the first to admit that I don't have the scholarship to know whether the Heter Mechira is a "solid" heter, or if the use of it should really be abandoned. However, one thing is clear, Rav Eliyashiv believes that we should not use the heter and keep the mitzvah of shvi'is (not working the land in the sabbatical year).

My question is this -- if using a Heter Mechira leads to the "desecration of the holy and precious mitzvah" of shvi'is, why doesn't the use of a prozbul lead to the "desecration of the holy and precious" of the mitzvah of shmitas k'safim (cancellation of debts)?

The Wolf