Why is B/V distinction common in Spanish songs?

As you may know in spoken Spanish B is pronounced the same than V (/β/). This is known as betacism and other than old fashioned teachers nobody encourages making the distinction. Furthermore, RAE says B/V distinction is incorrect but I've noticed that many singers, at least in Spain ,pronounce V like in English. This is not new, I've heard it to singers from 30 years ago at least and even there are ones like Marta Sanchez that force pronunciation of V and end up pronouncing it like German V or Spanish F. What is the cause?

Maybe it's a new trend, similar to pronouncing "th" as "f" or "v" in the trendiest forms of English.

This is so true. [v] shouldn't appear in Spanish at all (for either v or b), but it is so common in Spanish songs:

in this Lucero's song for example

http://www.youtube.com/watch?v=cZq24eCRCtc

''de mi vivir'' [demivi'vir] instead of [demi'βiβir]
''volar'' [vo'lar] instead of [bo'lar]