Whether a recall happens or not generally doesn't depend on the severity of the fault (except in really extreme cases); it depends on whether the manufacturer thinks they can get away with a small (for varying values of 'small')
PR hit if they don't institute a recall. When the
PR cost outweighs the cost of a recall, you have a recall.
In addition, having a recall involves admitting that there was something wrong; if there's nothing wrong with the original design, it's just that the new one is 'better', then why would you recall the old one? There may be legal ramifications to admitting fault, particularly in places like the USA. So the cost of a recall isn't just the cost of shipping and new bits.
The market for helicopters, even small, comparitively cheap ones, is pretty small. Also, the general public probably don't really care whether a bunch of rich buggers have to pay more to fix their toys. Hence, there probably won't be a recall, because Robinson can get away with making people pay instead.