[Plugin: WordPress SEO by Yoast] Meta keywords template, tag with comma?

hello thanks for this plugin, i have a question.. I use in my site the
Meta keywords template, and i use the template “%%tag%%”. It work but in the post, i see all tags without comma.. ex. if my tags are
“one, two, three” in see in generated page like meta keywords “one two three” without comma, for what reason?
Thanks!