Good questions. In this one, I don't believe that people "report" black bear sightings to anybody who officially cares (area biologists), and like Northwind writes, many (if not most) people who see a sasquatch keep mum about it, though that might be changing due to the nature of today's bigfoot media themes vrs what was televised 40 years ago.
Also, with reference to the claim that many people see a bear and mistake it for a sasquatch (which is clearly true): the opposite must also be true. People see a sasquatch and believe they've seen a bear. One obvious reason that would be true is because people have been conditioned to believe that sasquatches don't exist, so it must have been a bear. What else could it have been?
I'd like to reference you to some reading that might answer this question for you to some extent. First is the Glickman report. In it he specifically discusses sasquach reports, and how media exposure to the phenomenon effects reports, as well as hiw report densities are sure to differ regionally with respect to the population densities of both people and sasquatches:
http://www.photekimaging.com/Support/rptcol2.pdf
Secondly, here is the page for the Alaska Department of Fish and Game listing their management reports for all big game species in the state, including both black bears and brown bears (but not polar bears, which are federally managed). These reports can reveal a huge wealth of kniwledge and understanding about how official biologists estimate population densities for bears in the different habitats, in addition to so much more. If you're interested in big game, this kind of reading is addictive:
http://www.adfg.alaska.gov/index.cfm?adfg=librarypublications.wildlifemanagement