i don't think this is true. druids are more like a sect and they are independant from their races' alliances. take cenarion circle for example. they merrily help the horde pc if their interests are the same and also merrily kill the alliance pc if he/she was the aggressor against the horde pc and vice versa.
add baine's obvious despise to sylvanas. not the tauren but tauren druids would heed his call imo.