How can I restrict those connected apps with full access to my folders to the only folder to which they need access?
You can't. This is a choice made by the developer when they create their app. The developer chooses either App Folder access or Full Dropbox access, depending on what their app needs. You as a user can either agree to that access when connecting the app to your account, or choose not to use the app.
Thank you, Mark. I've voted for restricting connected apps to a specific folder.
However, with only 798 votes, it is unlikely to be addressed. And I think it is an important security issue that should be brought to the top of the list. It should not be the prerogative of the connected app developer whether they can access all files in someone's Dropbox.