Colin Holter has a post at the New Music Box noting Terry Riley's decision to now publish his scores through G. Schirmer. From what I understand, this decision comes out of Riley's preference, at this point in career and life, to get away from the everyday burdens of running his own publishing operation. In the far past, Riley made some of his scores rather widely available — with the original LP of In C including the score and Olson III available in a well-known anthology — but in general he held onto the music he performed himself or reserved for other musicians with whom he had a close, personal working arrangement and only in recent years committed the bulk of his notated works to his own publishing enterprise.
The decision for a composer not to be published for so many years by a conventional publisher has both practical and artistic grounds. Practically, the composer keeps all of his or her publishing and license fees (as opposed to forking over half to a publisher) and maintains a better overview of performances and subsequent fee collections, including exclusive performance rights (which are not unimportant when a composer makes a large part of his or her income from performing his or her own work; the whole issue of "cover" recordings is included here as is the question of a work's market saturation.) Artistically, it allows the composer considerably liberty to continue to define the contents, character, and performance practice of the work that a fixed and sold score might well discourage.
(At the end of Holter's post, he asks specifically about the works of La Monte Young. Having worked with Young in a publishing capacity before my time in Budapest, I can say a couple of things. The first is that although Young's control over access to his score may have frustrated many, Young has given a lot of thought about the matter and has had some very concrete experiences — many of them appallingly disrespectful towards both him and the work itself — with all aspects of the question and come to quite rational decisions about the optimal dissemination of the performance materials, based on a realistic assessment of the market for his work, his need to make an income for himself from his work, and his desire to maintain artistic control over work for which he has unique expectations — no shared programming, performance in specific physical environment, minimum durations and rehearsal periods, his own (or selected colleagues') participation in rehearsal, etc. — and which he clearly understands as very much still in-progress as both compositions and performance practices. Rest assured that Young has long-term plans for all of his works.)