This is a good question, as dictionary definitions don't generally explain how good people and their well-intended acts can be called infamous for the acts of others.
Ordinarily, someone or something "earns" infamy for doing or being bad or evil, and this is the definition given in most dictionaries1 and suggested by most thesauruses2.
However, in some cases one can become infamous for being the victim of a well-known bad act. Although I haven't been able to find this definition clearly articulated, it's easy to see how the more general definition can lead here. From MacMillan Dictionary:
well known for something bad
an infamous criminal
As with other dictionary entries, the example supports the idea that the infamous person or thing ought to be bad in itself. However, the basic definition lends itself to application to people or things that are famous for bad things that happen to them through no fault of their own. In fact, this usage is somewhat common. For example:
The novel Amalia was inspired by the historical case of [Argentinean dictator Juan Manuel de] Rosas's most infamous victim, Camila O'Gorman. (Lauren Rea, Argentine Serialised Radio Drama in the Infamous Decade, 1930–1943, 2016)
The infamous victim of abuse from her grandfather Josef Fritzl, who kept her in a dungeon in Austria until authorities found her recently, woke from a two-month coma by listening to Robbie Williams songs. ("Robbie Williams wakes Austria dungeon victim", NME, Jun 12, 2008)
The bloody slayings weren’t the only political incursion into those infamous Games. (Paul Hockenos, "An in-depth chronicle of terror at the 1972 Munich Olympics", The National, July 28, 2012)
That last example is very similar to the example in the OP: The 1972 Olympics themselves were intended to be a good thing, but because of an evil, violent attack perpetrated by outsiders they are now described as infamous. Another parallel is that we often use a descriptive and graphic nickname for the event that more directly refers to the evil acts, rather than the underlying event: The attack on the 1972 Olympics is often called "The Munich Massacre", and the attack on the first Selma-Montgomery march is often called "Bloody Sunday".
1 See, for example, Merriam-Webster ("1: having a reputation of the worst kind : notoriously evil 2: causing or bringing infamy : disgraceful"), Oxford Dictionaries ("1. Well known for some bad quality or deed 1.1 Wicked; abominable"), or Dictionary.com ("1. having an extremely bad reputation 2. deserving of or causing an evil reputation; shamefully malign; detestable"). Note that all of these also include a third definition, which is a technical legal definition referring to conviction of certain kinds of crimes and the resultant loss of legal rights. Although some formulations of this definition bear a superficial similarity to civil rights violations, that is not what this definition means.
2 See, e.g., Thesaurus.com.
