Just because Mr. Ashcroft can be sued does not mean that he will be be sued. And if he is sued, there is no guarantee that a judge and/or jury will find in favor of the defendant. Or, in the case of a jury trial there is the possibility that the judge may say any jury’s ruling is biased and reverse it in either direction.
The ruling is the result of an existing lawsuit against Ashcroft. The lawsuit by Abdullah al-Kidd accuses Ashcroft of violating his constitutional rights when he was held for for 16 days without charge in 2003 (he was held as a witness, and was never accused of or charged with anything). Ashcroft tried to get the lawsuit tossed, hence this ruling. al-Kidd’s lawsuit will now continue.