but no-one has objected to performers using arrangers with built in styles on stage for commercial gain, if anything they would be pleased for the free advertising.

Yamaha have objected to distribution of their styles by third parties, I'm guessing either because they fear the effect on sales of those products, or because they want control of the distribution themselves so you have to go to their web site.

The baldwin piano argument does not stand because there are no styles in a piano, you have to produce all musical results yourself. When using a style in an arranger keyboard someone else has already done more than half the work for you.

If you produced a million seller with their style and made a million dollars they may well have something to say about it, even if it was your melody? Now that would be an interesting test case...