According to this related Meta discussion, the ads tag was made a synonym of the advertising tag.
Though it seemed to be agreed there that ads was short for advertisement, there isn't a synonym between the two however. Ergo, it seems logical that advertisement should also become a synonym of advertising.
In regards to because I hate fun, please note that ...
I can see why these tags appear similar and should be merged, but there are some dissimilarities in the context of a Webmaster:
index can be used when referring to a file (such as the default
index file of a web site or web server). There are a few questions
which have used this tag in that context. For example,
indexing could reference ...