It appears they were journalists who entered as tourists, didn't disclose they were journalists, and were arrested either participating in or covering an anti-government riot.
What I found interesting is that "Bahrain is a small island nation
with a predominantly Shiite population ruled by a Sunni minority". Oil rich and just across the gulf from Iran this makes them the perfect target for covert or overt Iranian actions to support the oppressed Shiite majority. They are a closer and easier target and preliminary target before taking on Saudi Arabia.
Four Americans arrested in Bahrain - CNN.com