Why does wordpress charges money for this? Do they need to do something for this? Or it's just free money for them? Why can they offer free blog, but couldn't do so with domain mapping?
Due to how good Wordpress is I think it's fine. They need to pay for their free services some how. That seems to be a good way to do so.
I think $9.97 per year is reasonable price.
They offer free basic solution which is more then enough. If you need something additional that is the way to go
Or it's just free money for them?
If they're doing something, they've earned the money. There doesn't have to be an excuse for charging, or for a seemingly high markup. If you could pay $10 for the domain, you can probably pay $10 to point it to your free WP hosting.
I guess wordpress started charging as they become popular..that a generally trend.They word so hard to make wordpress interface and all so nice..they deserve that..
Depends on how much flexibility do you have after signing up their service.