Embarrassed to say that I assumed that tags hadn't been included (because of our poor results) and apologies for not checking.
The tags option is ticked.
I wonder whether the tag pages are SEO'd sufficiently for Google to enthusiastically grab them?
I have this same question. What should be included? I think that not including them would be better with all the clamping down Google has been doing lately?
You can toggle off what's included in the sitemap in the XML Sitemap Options page in the Admin CP. Just uncheck "Tags" if you don't wish for them to be included.