This is an unfortunate, but inevitable step that Yamaha is taking. However, some members appear to be overreacting.
As I read the letter, Yamaha gives consent to the use of their styles in songmaking, whether in audio or SMF format. In addition, Yamaha gives consent to the use of their styles in original stylemaking--as long as the modifications are significant. Presumably, the songs and original styles can be posted on the Internet. What is prohibited is the posting of Yamaha factory styles and factory styles modified only to the extent to make them more compatible with target arranger keyboards.
If Yamaha wants to prevent posting of Yamaha factory styles, they can do so in a variety of ways. They can close sites almost at will and they can start to employ technological means to prevent saving of factory styles to disk. It does not even matter whether Yamaha has the legal right to prevent distribution of styles--hosting sites will not look that deeply into the issue before closing their customer's sites. I think Yamaha is currently acting in its own enlightened self-interest, but I would not push it.
For those who remember the slaughter of the SMF sites, it is likely that some sharing of factory styles will continue. I bet you can post requests and have your friends respond by email. I think that it is the full-scale free-for-all posting that companies like Yamaha see as a threat.
In my mind, the style sharing community has only itself to blame for the restrictions being imposed. If style conversion were limited to out-of-production keyboards, Yamaha might never have acted. However, what was Yamaha supposed to tell its CVP-209 customers when they found their $7k styles freely available on their friends PSR-2000?
The bad news is not over. Technics (I think) has already acted on the issue by requesting that EMC StyleWorks not provide for conversion from their keyboards. (I certainly think this is a less desirable approach than Yamaha's.) Eventually all arranger companies will assert rights over their factory styles and prevent posting in original and factory form.
There may be a silver lining here. Maybe more custom styles will start appearing. Simon Phillips has made original styles available on the web, both for-free and for $. Maybe efforts like his will blossom once the free styles are removed from the arena.