It's mostly to avoid royalty payments for copyright material or performances. "Greensleeves" is about as out-of-copyright as you can get; commission some poor out-of work muso to record the piece for you for a one-off payment (or free ice-cream), and you own the performance copyright, so no further expense for multiple plays.

If you haven't seen the Bill Forsyth film "Comfort & Joy" (1984), distantly and humorously based on Glasgow's "Ice Cream Wars" (circa 1982) http://en.wikipedia.org/wiki/Glasgow_Ice_Cream_Wars it's worth a look - maybe not best viewed on YouTube Comfort and Joy - Part 1 of 9 (following parts can be found in the links to the right of the page) but it's scarce and expensive on eBay and Amazon.

Ross