She’s asking for “privacy”. Why did she tell anyone to begin with?
Maybe it’s to keep the leeching media away.
Because that way what gets out is HER story - not what the media makes up. Telling people that you have a certain diagnosis and asking for privacy (e.g., please stay away Paparazzi) are not mutually exclusive.
There are a lot of people in Hollywood who are in the business of outing the personal news of celebrities. Often, they'll demand a bribe to keep the lid on something. There's also people, a nurse for example, who would've been privileged to Applegate's condition and would have no problem selling tips to entertainment reporters.
She was diagnosed some time ago and might've figured the news was going to come out so she should just let it come out herself.