I certainly see your point, however, it does not take a "security expert" to go nosing through the browser's cache.
Even more data is stored in the browser's cache, a feature designed to speed up performance of websites by storing a copy of recently accessed information on a user's hard disk. InPrivate Browsing failed to disable this feature.
In all fairness this is a beta, it will be interesting to see if Microsoft at least fixes this problem in the final.
Ehtyar.