I’m not sure how to form a proper response because it seems you are both saying that Criminal Flags and low sec status are different, and at the same time saying low sec status is the same as an active criminal flag.
Unauthorized aggression in low or high security space generates a criminal flag. Criminal flags come with a set of consequences I can’t completely enumerate, but the most significant of those is death by CONCORD in high security space.
Losing enough sec status to bring you to -5 or less has a different set of effects not related to criminal flags at all. You are hunted by the Faction Police, a far less powerful force than CONCORD, and anyone who manages to shoot you won’t be punished by CONCORD. These players may be thought of as criminals, but from a game mechanics stand point, they do not have the criminal flag active.
The UI may make these two conditions seem to be the same thing by presenting both as flashy red on your overview so that you know some bad juju is going on, but they’re otherwise totally separate.
So, I gather that what you want to say is that people who have -5 or lower sec status shouldn’t be able to tether, but people still complained about the same when they’d just dock up. The structure could face whatever gate they wanted to camp and then they’d fire off in its direction straight away, what then? Deny them docking rights as well? (I know you did not suggest this, but I do not know where else this can go when just blocking tether ends up not satisfying the reason for the complaint.)
-5s can be proactively shot as soon as they land on grid with the gate. Jammed and warp disrupted, they’re sitting ducks for the faction police and/or gate guns. They seem to be sufficiently penalized to me. People just don’t find it worth their while to exercise the precautions necessary to contend with this weakened enemy and sometimes pay the price for it.
If they’re harassing players, that’s not a problem to address with changes to tether mechanics. That sort of inappropriate behavior should be reported when observed, regardless of whether the person is a ganker, miner, trader, or whatever so that CCP can keep the community clean and fun for all players on all sides of CONCORD law.