Reg, G4FGQ wrote:
"The broadcaster takes into account the revenue accruing from selling
the service."

Yes, and he takes into account the ease of compliance with FCC Riles in
the USA.

Although 90 radials is a minimum requirement for the U.S. AM
broadcaster, here is the rule quoted by John Edward Cunningham in his
1977 "The Complete Broadcast Antenna Handbook", from page 311:

"The current FCC Rules specify that the radials should be at least 1/4
wavelength long and that there should be as many as practicable, but in
no case less than 90. The Rules add that a system of 120 radials spaced
every 3-degrees and extending 0.35-0.4 wavelength from the tower is
considered an excellent ground system. In addition, a square ground
screen 24 or 48 on a side is often provided at the base of the tower,
particularly when the tower height is such as to cause a high voltage.

Whenever a less than optimum ground system is used, the FCC requires a
complete field-intensity survey to establish that the effective field at
one mile meets minimum requirements."

Best regards, Richard Harrison, KB5WZI