1) Perceived Risk. AI makes any activity 10 times scarier than same activity without. People will irrationally fear that their AI PA turns into HAL 9000[1] or their self-driving car turns into Project Satan[2].

2) Actual Risk. Software has bugs. When software is meant to give out some subset of data it is not unknown for it to give out more data than it means to. A human PA may make mistakes and blurb something that he/she shouldn't have, but he/she also knows when to stop giving out information. If a particularly trained AI starts leaking out information, it will happily do so for user after user after user.

[1] http://en.wikipedia.org/wiki/HAL_9000 [2] http://futurama.wikia.com/wiki/Project_Satan