The treatment the Boston Massacre gets in modern high school textbooks would imply that the event was closely related to the events of Lexington and Concord, or even the Intolerable Acts-- events which occurred four or five years after the fact.
In my view, this is actually a pretty long stretch of time to associate the events so closely. Five years ago, the world was a very different place. I'm sure the same was true in 18th century Colonial America (i.e. five years is a pretty long period). On a contemporary level, it would mean we could tie the events of Ferguson to the events of Occupy-- though, in reality, they have very little connection aside from the fact they revolve around questions of poverty and police violence.
So... why does the Boston Massacre receive the treatment it does, especially when the legal outcome at the time was that it was justified?
It would be helpful if you could elaborate on what these "high school textbooks" say about the connection between the Boston Massacre and Lexington/Concord. "Closely related" could mean many things; it's hard to make a rebuttal to such a vaguely-worded claim.
Most historians would go even further and say that the Proclamation Line (1763) and the Stamp Act (1765) were closely related to the eruption of violence as well. The American Revolution was a process which reshaped the relationship between the colonists and the British, and this critical period was a good dozen years long. At the end of the Seven Years' War, the colonists toasted the health of the king; in 1776 they declared independence. The slow drip of new and unprecedented legislation strained the familial ties to the breaking point. There were a host of issues during these years which can be said to directly relate to the outbreak of armed hostilities. The colonists themselves, in their own minds, certainly tied these issues together and became convinced that they were part of a nasty plot to gradually reduce the colonists to slavery. They saw a pattern.
The Boston Massacre was part of this pattern, and the bloodshed made it unusual and memorable. The legal outcome mattered to some colonists, but certainly not all. There were considerable class divisions during these years regarding which grievances were most important, and how best to solve the crisis. The upper classes disliked British rule for different reasons than the lower classes. Boston in particular was a town wracked with poverty and unemployment in 1770. The soldiers were competing with the townsfolk for jobs. Sailors, who made up much of the mob, were being impressed by the British. Even during the Seven Years' War, the cultural differences between the British soldiers and the colonists were apparent, and the Bostonians perceived that the soldiers were insulting them.
The trial was meant to send a message to the British and to fence-sitting Americans that the mob was not representative of the colonists. It was part of the propaganda war between the Patriots and the Loyalists. Moderate Patriots did not want to give the impression that they were a bunch of hotheaded ruffians. They needed to show the British that they deserved negotiation, and they needed to demonstrate to uncertain Americans that the Patriots were genteel. However, the verdict didn't resolve the very real grievances of radical lower class Bostonians, or draw them any closer to the moderates.
Another way to answer your question would be that the winners write the history. That is, American nationalists have attempted to make the argument that the Revolution was justified, and their evidence has been the laundry list of bad things the British did in the lead-up to the war. The Massacre has been a good propaganda tool for nationalists, and historians (but not all) have participated in this project.