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It's because the city code since 1974 has required helipads on top of tall buildings. (Luckily for the First Interstate Building, circa 1988.) But the flat-top rule may be waived, along with a lot of other laws and regulations apparently, to promote the controversial construction of the first skyscrapers in Hollywood, under the proposed new community plan for the subway corridor along Hollywood Boulevard. From Jeremy Rosenberg's Laws That Shaped L.A. feature on KCET's website:
"If this provision in the Hollywood Community Plan survives," Los Angeles Planning Commissioner Michael K. Woo said in an email last Friday, "then that will be very significant."Woo, the Dean of the College of Environmental Design at Cal Poly Pomona and a former L.A. City Councilmember, would know. He is in great part responsible for this potentially transformative progress, having proposed the change to the City Planning Department staff's initial report.
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"It's something that I've thought about for at least twenty years," Woo told [Rosenberg] in 2008. "Just because it was so obvious that Los Angeles lacked the kind of dazzling architectural tops that you see in other cities."
Regarding the First Interstate tower, now the Aon Building: the tallest structure in Los Angeles at the time might have had a spiffy helipad, but what it really needed was sprinklers.
LA Observed photo of Bunker Hill skyline