This question about why the first Selma-Montgomery march is called "infamous" was put on hold a day or two ago for lack of research. The asker has now quoted the dictionary definition that was consulted, including a link (dictionary.com).
I don't think that this is a case where consulting the dictionary definition helps—in fact, rather the opposite. While it's true that the word infamous is regularly used for events and locations that are not, in themselves, evil, most dictionary definitions strongly suggest or even flat-out state that the word is used for people or events that are "bad" either inherently or in action. Examples tend in this direction, as well. For example, the definition that pops up from a Google search is (emphasis mine):
well known for some bad quality or deed.
"an infamous war criminal"
synonyms: notorious, disreputable; More
- wicked; abominable.
"the medical council disqualified him for infamous misconduct"
(A third, technical legal definition is even more confusing.)
Given this emphasis in dictionaries on badness of the thing described as infamous, it seems reasonable to me that someone would be confused about whether it is appropriate to call an event that was attacked, rather than the attackers, infamous.
TL;DR: The questioner said early on that they were confused about the definition, and has since added the requested definition, and has stated what is confusing about that definition vis-à-vis the specific usage. I think this meets all of our requirements for an on-topic question.