When proposing a synonym one need to comply to:
Users with more than 2500 reputation and a total answer score of 5 or more on the tag, can suggest tag synonyms. Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms. Suggestions will be automatically approved when they reach a score of 4, and automatically deleted when they reach a score of -2.
I agree with the reputation requirement: it makes sure that the user is aware of the (inner) workings of the site.
I suppose that this reasoning is similar to the tag score requirement: one needs to provide (s)he has knowledge about the tag.
But I do not agree with this second (tag score) requirement.
Without knowledge of the tag subject it is not a problem to detect synonyms (for most of the tags):
jpg vs jpeg: easy, both are almost the same: jpg is a file extension of JPEG, but we do not require a separate tag for each file extension of a JPEG file)
koala vs koala-gem: easy, koala is a Ruby gem (according to its tag wiki) and thus is exactly the same as koala-gem (I am (now) aware that there is a language in development called koala, but koala is not about that language)
lots of singular vs plural synonyms: easy, I suppose that this does not require further details
All of these above requests (and there are loads more) can be done by a trusted user without any knowledge on the actual topics.
So, I do not really understand the tag score requirement and would propose to remove the tag score requirement.
Additional thought: If the moderators are afraid of misuse of the synonym request system, the reputation requirement could be increased instead, so only 'more responsible' users have the permission to propose a synonym (although I do not think it is necessary to increase the reputation requirement further).
Edit: Daniel Fischer came with an improved proposal: Low reputation users keep the tag score requirement and high reputation users do not require it. So low-reputation users are still able to suggest synonyms for the tags they have enough knowledge and high reputation users are trusted enough to suggest any tag synonym.