Having to have had to deal with drunk passengers, the crew was correct in asking for assistance. You don't know what some of these drunks will do. There have been cases where they have tried to start fires in the lav, try to open emergency or main exit doors, try to get into the cockpit, etc. We were not there, so we are not totally informed on what happened.
First of all, the aircraft was a US Flagged aircraft, so under international law the Marshals had authority to act. Quite likely a FA asked the passenger to leave the galley and return to her seat. When the woman refused, the FA asked the Marshals to act. It is my understanding that they will normally ask the passenger to return to their seat or face arrest. According to the information, the woman attacked the Marshals and bit one of them. As for having 2 Marshals handle the situation, it is a standard law enforcement method. Almost any LEO will tell you, the most important thing is to go home in one piece at the end of the shift. The next is get the job done. And this is most likely accomplished by avoiding one on one. Over powering force is the order of the day.
It was definitely a political play, as the Marshals were turning the woman over to local law enforcement. If it was a real issue, all the locals had to do was release the woman. Personally, I feel the Marshals were correct in their actions and did the right thing in leaving the country. From what I read the signs were there that they were going to get really ******.