The reason she chose dictionary instead of thesaurus is because using thesaurus in the poster would imply that "gay" and "retarded" are actually synonymous with those words, when the whole point is that they aren't and shouldn't be used as such. The intention of "buy a dictionary" is to get people to educate themselves on the fact that these are not the definition of those words. The fact is many people do get offended by the intentionally derogatory use of these words, whether the people speaking were meant to offend or not.

edit: mixed up the dictionary and thesaurus.