They do pass
PR, but only for the Toolbar. There is a total amount of
PR of course. But when the search is done and the results are ranked, not all the links are counted, and with that not all
PR is counted. So
PR for Dell for the keyword computer is huge. But for the keyword "rembrand painting" it is 0. Even if Dell would put that phrase in a page, they still wouldn't rank high for it.
If anchor text wouldn't be used like this, all results would always be high
PR sites.