CW's right. It's not illegal, it's just stupid. Mentioning the competition directly, whether it is good or bad, is just another plug for them. No matter if it is good or bad, you don't give free publicity. Also, you don't make yourself look better, by making your competition look worse.
I doubt it...
Examples 96rock "Kiss Sucks" -- Kiss flipped from oldies to rock, and Inner City sold 760am to Clear Channel in 1993 Urban AC was hot in the early 90s so bye bye 96rock.
KZDC "995 kiss sucks, KZDC rocks" --Lasted until sold to BMP radio
Several slogans from K-rock 102.7 -- Lasted until CBS flipped it due to Sterns leaving CBS.
977 Rock- Kiss keeps talking blah blah blah blah blah and we rock
If was not successful, they would not do it. I guess if more than 2 stations due it, it must be successful.
That is a funny question "is it legal?" I wonder if the OP has ever watch polictal ads and debates. Legal yes, moral probally not but we are not tallking about a religious station here are we?
And, 93Q is still mentioning KILT.