Wording of Berachot

[rhu]

So in yesterday's Daf Yomi, the Talmud spent a bit of time discussing when the bracha that is recited when performing a mitzvah takes the form "...who has commanded us TO DO X" versus "...who has commanded us CONCERNING X". The explanation that was given was that we use the "to do X" form when:

• The mitzvah is compulsory, not optional
• The mitzvah cannot be done via an agent (shaliach)
• The mitzvah must be done at a specific time

For example, one says "al hashchitah" rather than "lishchot" when killing an animal for food, because one has the option of eating something else. One says "al hamilah" rather than "limol" because it's usually the mohel performing the circumcision on one's son, not the parent. And one says "al biur chametz" rather than "liv'or chametz" because the destruction of chametz can be done any time prior to Passover, even up to the day after Passover ended the previous year.

So here's what I'm wondering. At the seder, when we fulfil the mandatory mitzvah of personally eating matzah between the "time of the stars" and midnight on the eve of the 15th of Nisan, why do we say "al achilat matzah" and not "le'echol matzah"? (I.e., "...who has commanded us concerning the eating of matzah" and not "...who has commanded us to eat matzah")