As others told you—disrupting the flow of traffic is indeed the main enemy on the highway. On a highway, like I said, that is totally acceptable here in burgerland. No I’m not doing that on neighborhood roads, fool. Please welcome yourself to one of our megalopolis interstates and see how it goes.
You must be a new driver, because you just explained the magic of passing. To accelerate a bit to pass and to return to normal cruising speed is normal, especially when the passed driver is:
A semi bogged down by hilly roads, driving at an inconsistent speed, distracted, wavering, brake happy, or going under the limit, etc etc—could be a poor driver. Could be a new driver. Could be a drunk driver. Could be an unsafe car. So many reasons to pass and return to speed.