I think they're right - it's almost all due to stylistic changes.
Just went down to the cellar and looked at a few randomly pulled bottles of Monte Bello - 1995 = 13.3; 2008 = 13.5; 2009 = 13.5. Zins are higher but one made me look twice - 1983 at 12%.
Looked at some random Dunns too, from 1987 thru 199 and they're consistently 13%.
There are many more wineries now than there were 20 years ago, and if people have a preference for riper wines, that's one reason for the average alcohol level to be higher. But more importantly, I think the style has become far more oriented towards ripe and lush wines.
Grgich changed right around 1994 - look at the bottles from the early 1990s vs those from mid 1990s. B.V. has changed w/in the past few years and when they engaged Rolland in 2007, Parker announced that they were "back" as a top notch producer.
I don't think it's "catering to the Parkerization of the modern palate" at all though. I think he likes what many people like, it's not like people "naturally" would like thin, green, weedy wines if it weren't for his nefarious influence. I think winemakers finally figured out how to make lusher and riper wines and the critics and public have responded positively.
More importantly, I think Pape du Neuf is right on the money -
Lack of tradition.- No one ever defined what California wines should taste like, giving plenty of freedom for expression and variation.
In the time I have been drinking California wine the pendulum has swung from austere to ripe to austere and back several times.
People are still figuring out what's "best". I hope nobody ever does define what CA wines "should" taste like so that the pendulum can keep swinging.
Personally I don't like hot, sweet wines, but sometimes you just don't notice the alcohol at all. Shafer HSS is an example.