According to the results of a recent meta vote questions about identifying music videos or commercials have been ruled off-topic (and for the sake of this discussion here, that is to be taken as given and discussed elsewhere if you feel the need to). But as a result of this decision the corresponding tags identify-this-music-video and identify-this-commercial have been made synonyms of identify-this-movie and identify-this-tv-show respectively, instead of just deleting them (or leaving them questionless to further automatic deletion), which I find highly counterintuitive.
Given that those tags pertain to invalid question categories, their role as synonyms for valid question categories is somewhat questionable. This on the one hand shows that those kinds of questions are on-topic and a sub-category of movie/TV-show identification, which is certainly not the case. Furthermore, it encourages users to ask such questions in the future. While nothing stops users to ask such questions even without the tag, and it's always possible to explain the true situation with the help of the above mentioned meta discussion anyway, such a tag still actively encourages those questions and serves as "proof" that they're a valid on-topic question category. This is not the purpose of a tag synonym, which is to be an alternative name or valid sub-category of another tag, but not to automatically pose invalid question types as valid ones.
While of course the above fears about future problems might very well be ungrounded in practice and those disadvantages might just be hypothetical ones, I fail to see the actual advantages those tag synonyms bring. In addition to that the problem this brings for the structural integrity of the site and its tagging system does not need a precedence case to be an actual problem. So I would like to question the arguments in favour of making those invalid tags synonyms of valid ones instead of deleting them and the reasoning behind this decision.