Yes, connecting a directional antenna is illegal because its gain "amplifies" the power in a given direction at the expense of all other directions. We of course didn't tell anyone, and back then there weren't many services that we could disrupt by pointing the antenna from the 9th floor of a building to a nearby hill:)
Using directional antennas is legal if you compensate by turning down the transmission power so that in the direction you are still transmitting the signal is no more intense than it would have been with omnidirectional antennas. That doesn't entirely defeat the purpose of using directional antennas because it still means that the receive side is focused specifically at the direction of interest and not picking up as much noise from irrelevant directions.
It's not illegal if you have a ham license, though. This allows for interesting hacks such as AREDN, which uses off-the-shelf WiFi hardware to run a mesh network, using high-gain (and usually directional) antennas to link larger nodes to each other:
