First of all, I would like to make it clear that this is not very important and I agree that meta tags are not very useful, but they have some cosmetic yet real usages too.
Anyway, we can find this in both provsilver and subsilver's overall_header.html :
This, is addition to being useless, makes it impossible for extension to set up these meta tags as wished using the overall_header_head_append event.
I don't think phpBB should make such restriction, so I suggest to remove these too lines or at least to make them conditional with a template variable.
I originally posted here : https://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45907
But I though it may finally be more appropriate to fill a bug report.