This question already has an answer here:
(Why are accepted answers not included in the tag badge calculation? has a different focus than this. That one explicitly asks solely for the rationale behind the current design. Arguing about the design, both in favor and possibly against it -- is thus prohibited there as off topic.)
Noting that accepting and upvoting are intersecting functionalities -- both mark an answer as useful -- I wonder if accept marks should count towards tag scores as well. The purpose of a tag score is to measure a user's contribution to the tag, and they definitely both signify such contribution.
- Here's a litmus test for if an acceptance mark is fundamentally different from an upvote or not: "Would you think that a vote by the OP should be excluded from tag score if there was no separate acceptance mechanic?"
One could argue that an OP would upvote a useful answer as well anyway, but this is by no means mandated by the system. Moreover, if an OP is a novice, they even can't upvote at all -- which thus unfairly disadvantages those answering questions by newcomers.
AFAICS, the primary motivation for the current state of affairs is solely the implementation simplicity -- which is by no means a justification for perpetuating a flaw.
Sure, if an OP both upvoted and accepted, it should probably still count as one point. This is fairly easily doable, e.g. by only counting the accept mark if the OP is not among the upvoters (or by subtracting one if they are).