[tag:C] in a question or chat produces a nice c link and visual representation of the tag in question. The case gets lost, but it's clear that it's a tag and that this was the reason for the loss. Doing the same in a comment gives something more akin to c, i.e. the case is removed and there is nothing to distinguish it from just another hyperlink. I'll add an example comment to this question just to demonstrate.
The loss of case without any other presentational gain to offset it makes things look low quality (poor capitalisation) or just plain wrong in some cases.
Represent tags in comments better
At least don't lose the case - it conveys a lot of meaning sometimes (e.g. LaTeX).
The present situation of pretending to honour the notation, but doing nothing other than stripping the case from the string is worse than not honouring it.