For the record, I think every All-News stations in the 70s and maybe 80s used the teletype machine sound effects. That included CBS stations such as 590 WEEI Boston and 880 WCBS New York, plus the three All-News stations owned by Westinghouse, 980 KFWB Los Angeles, 1010 WINS New York and 1060 KYW Philadelphia. WINS and KYW continue to use the teletype machine sound effect today, although the other All-News stations have dropped it.
I've heard it said that due to WEEI's limited 5000 watt signal (plus the proximity to 580 in Worcester that also limits its coverage) CBS decided to sell the station in the 80s. All the other CBS All-News stations at the time were 50,000 watts except 950 WWJ Detroit, which later upgraded to 50,000 watts. CBS later merged with Westinghouse, bringing its four All-News stations (including 670 WMAQ Chicago) under the CBS banner. So that left 980 KFWB Los Angeles as the only CBS All-News station not running 50,000 watts. And in the last decade, All-News was phased out at KFWB, except for morning drive, as CBS plans to sell it off.
As said above, All-News stations in markets smaller than Boston including Detroit, Seattle, Ottawa, Calgary and Edmonton, go 24/7 with the News format. It's only a quirk how WBZ developed into an All-News station from a Full-Service AC station that had nighttime Talk shows. I suppose David Brudnoy's evening Talk show was so popular, and news-oriented, that CBS kept WBZ doing Talk shows at night, even after the switch to All-News in the daytime. Brudnoy used to say his show was #1 in the evening, so he never got replaced by News. And that kept the overnight Talk show in place as well.