Many European countries accept U.S. travellers without any additional filings because they trust U.S. passport holders, generally speaking (except the ones they have files on, in which case they'll already be following that up when you get there). The U.S. does not necessarily trust Swedish passport holders in the same way that Sweden trusts U.S. passport holders.

Wouldn't that be Eastern countries, technically speaking? unless you used it as a synonym for "First" world countries.