They probably are, but not to the same extent.
Siri differs in two crucial respects:
1. Bing and Google don't, by default, tie searches to an individual. (Yes, I know, they can, you can log in, and sometimes are already, but you can use both services with cookies turned off without problems.)
2. Siri searches your personal information. At least, that's what I figure from the ads. If Samuel L. Motherfucking Jackson can cancel his golf game by telling Siri to cancel it, then clearly Siri knows SLMFJ's schedule, amongst other things. Google and Bing, unless your business uses Office 365 or Google Apps (in which case...), only has limited personal information on you.
I'm not arguing they're not potential security holes, but they're not in the same ballpark as Siri. If you're talking to Siri all the time, in order to modify your work schedules, send emails, etc, then, well, you are passing much, much, more information to Apple.