In addition to gbeatty's excellent answer, I would add an additional reason for the presence of the Romans:  the contrast to their rule and that of Jesus.  It is Jesus who de-emphasizes the rule of both the military and the rabbis, and while he recognizes the power of men on earth, he asks Daniel and all of those who would follow him to not become overly concerned with the rule of man but to focus instead on the will of God (for which, of course, he was an ambassador.)  There are, as Daniel discovers, both good and bad Romans, good and bad rabbis, good and bad men and women, period.  Only the purity of Jesus stands in contrast to the flawed reality of mankind. 

In The Bronze Bow, the Romans play several roles. First and most simply, they are who Daniel hates. He hates them because he sees them as oppressors of his people. That's also why the band of guerilla fighters live in the hills. As conquerors, the Romans are seen as the enemy, and so in addition to being seen as wrong or oppressive, they are seen as the establishment. Together, these functions allow them to force Daniel and others into their own characterization. To oppose the Romans is to fight for freedom, to be a hero, and to be original—a rebel. This means that the Romans play a role in the characterization of Jesus; he must be seen in opposition to the existing powers.

However, while the Romans do arrest people in the book, and do rule as occupying military forces, Speare does not paint them all as evil, and so they must be seen as realistic secular forces.