The central question seems to be the following, articulated most precisely by Mike Lee (R-UT): even if the organic search results are entirely unbiased, Google also programmatically (via "Google Universal Search", the system that organizes the overall search page) inserts specific links to other Google pages--such as Places, Maps, Shopping, or News--in very prominent positions. There are competitors for all of those sites (Yelp, Nextag), but Google systematically (it seems) prefers their own version.
This behavior is relevant to an antitrust principle that a firm with a monopoly in one area/market should not be allowed to use that monopoly to get inordinate market share in other markets.
The senators do not appear to have concluded that Google is doing anything illegal, though several have used the words to the effect of "I'm concerned with what is happening here."
My take: I agree with other commenters here that the senators have at several points seemed to have gaps in their understanding of Google vs. the average HN commenter, though I didn't hear any overtly wrong claims with respect to the central question. Maybe this analysis is better done by the FTC (there is an investigation in progress there), but it does not seem ridiculous that Congress, which is somewhat more transparent, is asking these particular questions, which do not seem incoherent or pointless.