Charging for access is common when some project is wanting to access a large amount of data. Commercial or otherwise, scrapers take up forum resources that are greater than the typical user, likely by a lot.
Probably. But (at least the known) scrapers give something back to the community.
Scrapers also have zero chance of even looking at any ads that are served upon forum users, which negatively affects forum revenue.
Users can disable ads too. But there are no ads anymore, the forum won't need money any time soon.
One of the things I like about Bitcointalk is this:
the forum's mission to be as free as possible.
I'd say scraping is part of that. The "1 page per second" rule still applies after whitelisting.