Frank's career was plagued by bootlegging and piracy. His international audience made it easy for anyone who could press a record (or later copy a tape) could sell Zappa shows and sometimes even albums. There's a reference on Just Another Band From LA to the Live at The Fillmore East album being heavily pirated. Frank recorded some of his shows and Jimmy Carl Black is widely thought to have stolen and sold some of those as well as studio bootleg material. Apparently, if you lived in Europe or New York City there was a huge array of Zappa's work available on the black market.
Now, Frank was never rich because it was expensive to tour with the large bands he preferred. According to his book transcription costs nearly broke him more than once and hiring orchestras was fraught with peril. The idea of other people making money off of his work really pissed him off. He prosecuted some and trash talked on others but it took until 1991 before he finally got his revenge.
That's when Rhino released Beat The Boots 1, a career spanning retrospective of several popular bootlegs. The discs were available individually and as an LP or cassette box set. The project was successful enough that another box set was released the following year. In most cases the released versions of these official versions were just straight copies of the bootlegs but the fans seemed happy enough to buy them. Later, to reiterate the point in the digital age the Zappa Family Trust put out Beat The Boots III on iTunes last year.
Now Frank didn't really want the bootlegs out there at all. He was a perfectionist who was very rarely satisfied with the final recorded albums so the idea of people wanting a bunch of crappy bootlegs was kind of insulting, but not as insulting as someone else profiting from them.
It is worth noting that the Zappa Family Trust has released 18 albums, many of them professionally recorded live material or extensive studio experiments that Frank recorded but never used. If you're looking for a Zappa album you've never heard there's probably one out now.
Zappa Radio from Zappa.com