If there is a little "add me to your type pad or Blogger" link on the side then I just do it. I take that as an open invite...KWIM.
other than that I just use my judgment. I have asked a few ladies...but generally If its a public blog and you read it then IMO its something you are sharing about you on your blog... your reading list so to speak and OK to add whomever you DO read. that being said...if someone was offended by me adding them I wouldn't hesitate to remove them. I honestly use my link list to view blogs I frequent. Helps my browser favorites stay organized where I want it them to be!
Hmmmm...maybe they want it to be more private and don't know how to make it that way in their Blog Service.