Doubt it, it's more likely that they want to maintain full control over results. They likely have certain results they don't want you to filter. Also, relevance can change over time. You might filter out something now that made sense to filter out now but not in the future after the site changed and got reindexed.
right, their value proposition to you, as a user of their search engine, is that if you type in a few terms they'll present a carefully curated list of locations on the internet that are not only relevant to your search terms but also are useful to you, specifically, at that moment.
supporting user controlled blocklists would implicitly acknowledge that their system can miss the mark. i suspect they focus on not missing the mark rather than providing supplies for their users to paper over problems with their system.
i'd be really surprised if there are "certain results they don't want you to filter." separation of editorial and business in publishing has been around for a very long time and it would be pretty hard for them to defend if it came out that they were using their own ad revenue as an input to how they rank and filter results.
edit: i would also suggest that if they were boosting results that drove ad revenue for them, they'd have been caught by now. there are a lot of people out there who make a living experimenting with how to boost a google ranking. if it was as simple as "use google as an advertising partner on your site" i'm sure we'd be hearing about it.
