Looking at the last 10 retag request that were completed but did not contain the synonym request tag, 8 resulted in a synonym (3 of which had synonym in the title) and 2 did not. The two that didn't asked about whether it should be synonym request as well.
This leads me to believe that most people expect that if their request is granted it will also be blessed with a synonym, and even though the mechanisms are different I think the subtlety is usually lost on the people making the request.
Furthermore why would one make such a request? Its like going "Hey there's a weird state with out data, but lets leave it so the weird state can be re-introduced!"
Resulted in a synonym
Did not result in a synonym
As Brock points out in the question's comments, there seems to be a fundamental reason for having two separate tags. While implementation-wise merging tags may well be 'simply a different flavor of retag-request' (tag-wiki), in practice there are entirely different social processes involved when creating synonyms and when retagging.
If many people are using [retag-request] when they actually mean to request a [synonym-request], then we could perhaps raise another question: How do we encourage people to use the appropriate tags when retagging and synonimizing tags? I would start by including some of what Brock mentioned in the tag wiki.