In that link, someone compares Angel turning back time in I Will Remember You and not telling Buffy to Spike getting a soul and not telling Buffy.
There is one pretty significant difference here, which I did not see anyone pick up on. In I Will Remember You, Buffy and Angel are in a relationship. They are sleeping together, and Buffy calls him her boyfriend. This makes major decisions - like him choosing to go back to his vampire self - her business. By contrast, in late S6 Buffy and Spike are very much NOT in a relationship. She has made it very clear to him that they are extremely over. So, even though he did it for her, Spike going to get a soul is not really her business.