if the last slot was used by a hacker, there was one less slot for a paying customer. ... unless AT&T was building more capacity to support the hacked phone calls, then there was really no real cost to them (except maybe termination charges for international calls)
But the network traffic, like power consumption, varied a lot with time-of-day, and the network had to be sized to handle the peaks. The phone phreaks usually did their deeds at off-peak hours.
Even if they DID have to install extra equipment, that just meant they made MORE money. The arrangement that granted their monopoly, in return for providing universal service, let them (in cooperation with the regulatory bodies) set prices so they received a guaranteed rate of return. The more they spent, the more profit they made. So as long as the phreaks weren't disrupting things too badly they weren't a financial drag.
That, by the way, is apparently the genesis of Bell Labs. As long as they spent money on something plausibly related to improving telephone service, every dollar they spent brought in a dollar and six cents or so. So Bell hired a lot of smart people, gave them equipment, and told them to go to it (and just publish a couple articles a year in the company journal). (Financially, though, it was a "failure": Chartered to lose money, it actually made money, even in its first year, by licensing the technology it developed.)