Even though the speaker knew he shouldn't eat the plums, and probably knew he would feel guilty afterwards, he decided to eat them anyway. That, Shmoopers, is a choice. But we don't really get to see him make this initial decision. Instead we read about him dealing with the aftermath. And in dealing with it, he makes yet another choice. Our speaker, in writing "This Is Just To Say," has decided to apologize, poetically.
The speaker's major decision in this poem is not whether or not to eat the plums, but whether or not, and how, to apologize.
The speaker's decision to include a description of the plums shows that he wants the person who missed out on eating them to enjoy their tastiness. Except they can't, Mr. Speaker. You ate them. Sheesh.