Because abuse by a very few destroys trust in and value of the system to all:
[S]ince mid-2015, a consortium of engineers from phone carriers and others in the telecom industry have worked on a way to [stop call-spoofing], worried that spam phone calls could eventually endanger the whole system. “We’re getting to the point where nobody trusts the phone network,” says Jim McEachern, principal technologist at the Alliance for Telecommunications Industry Solutions (ATIS.) “When they stop trusting the phone network, they stop using it.”
That’s not a bad answer (“things that undermine trust in systems are bad”) but when generalized, can lead to undesirable results. For example using your axiom, undermining trust in a corrupt and harmful system would also be bad. Like for instance, speaking out against a repressive government regime.
Trust is one element. Common weal, the sense of acting in the common good, is the more critical element.
Trust here is a mechanism, rather than and end goal itself. Through the undermining of trust in a system for the public benefit, the common good is harmed.
Your counterexample becomes far less difficult when phrased equivalently:
Through the undermining of trust within a system opposed to the public benefit, the common good is increased.
TL;DR: you've confused ends and means.
Though generally, common good is extremely reliant on a strong social trust fabric.
(As to the question of whether or not "common good" / "common weal" is an appropriate goal value of a system, that's another discussion. I've elected to adopt for now the general idea that it is, but there are varying points of view on this point, and cases in which there is no globally beneficial outcome. Most especially where some are disadvantaged such that others, any, might survive. See also: "lifeboat ethics".)
[S]ince mid-2015, a consortium of engineers from phone carriers and others in the telecom industry have worked on a way to [stop call-spoofing], worried that spam phone calls could eventually endanger the whole system. “We’re getting to the point where nobody trusts the phone network,” says Jim McEachern, principal technologist at the Alliance for Telecommunications Industry Solutions (ATIS.) “When they stop trusting the phone network, they stop using it.”
https://nymag.com/intelligencer/2018/05/how-to-stop-spam-rob...