Why Does The Onion Find Sexual Slavery Hilarious?

Fans (and non-fans) of The Onion may be interested to see this NEW! item they are selling at The Onion Store:

The shirt says "My friend went to Thailand and all I got was this lousy kidnapped prostitute." Note the helpful disclaimer: "*Prostitute Not Included." Hmm. Must be "ironic."

Notice that the catalog entry just says "all I got was this lousy prostitute", but the shirt itself refers specifically to a "kidnapped prostitute."