Well, the odds are good that they were simply doing their job. The public utilities and power companies sign covenants with the land owners and those are normally on file. If one keeps the area under power lines mowed, there is no need for them to spray. Typically, there is a "no grow" policy, especially if the power lines are carrier lines, providing power up stream.
We have the same issue here. I organically garden and one patch is under the power lines. The power company regularly sprays my neighbors, especially if they fail to brush hog often enough to prevent sapling growth. It is far cheaper for them to spray than it is to wait every 10-15 years and have to send in a de-foresting crew and equipment.
So, I guess the bottom line is be sure the power company has an easement covering the power lines across your property. Be sure the size of the easement isn't larger than one might think. My neighbor was shocked to discover a 200' wide easement existed. He bought the property without doing his due diligence. Also be sure the power lines are not located in the road right of way, which is also common. In these cases, there is no covenant, as none is needed, the road right of way exists.
All that said, you'll find discussion with power company supervisors often work well. If you agree to keep the right of way clear and mowed, no saplings, no trees, no stumps growing, etc, they'll usually mark their file and the pre-spray team that surveys will see that there is no need to spray.
Finally, if the crew was indiscriminately spraying beyond the right of way, and you are sure of the right of way precisely, then you should absolutely call a supervisor at the power company and express your displeasure at the spraying on your property. Hope my long reply is of some help.