Calls and texts are of course cheap on a per-unit basis, but they're still not free. So not only it means that in aggregate, telcos will lose maybe 10% of reven

Calls and texts are of course cheap on a per-unit basis, but they're still not free. So not only it means that in aggregate, telcos will lose maybe 10% of revenue if they do fight it, but most importantly that detail actually means every bit of traffic has a paper trail associated to it (since this traffic must be billed for) and thus all the information required to trace the source of the spam is there, just that everyone (including regulators) are financially incentivized to look away and pretend it isn't.