Directional antennas still produce sidelobes. They aren’t laser precise. Keep in mind radio telescopes are very sensitive, a satellite like BlueWalker 3 would be far stronger than natural radio sources in the sky even if it isn’t directly aimed at it… and the goal is to have enough of them to maintain coverage of an area at all times. The point is to at least not directly aim the beam at a radio telescope, and provide a little extra buffer room to account for the sidelobes.