Surely if this is so, why haven't BMW called in all Z4 and replaced faulty coil springs or improved the quality of the coil springs. From reading your pages this has been a probllem with Z4s from day one.
They wont recall if they dont have to, as they then have to publicly record the fact. They are playing the numbers game, it is either a design, a material or material processing issue that did not get captured until the first recorded failure, which would mean 1000's are affected. They will look at the FMEA to see if it is a safety issue or not, and then decide to either recall, service retrofit or fit on failure, the first one obviously will put the supplier out of business, the latter two will be dealt with by warranty claims on the supplier, hence why they fit them free of charge even outside warranty. It is cheaper to do so.
I have worked on numerous potential issues on vehicles, luckily for the suppliers involved they have not led to recalls, but they have either been dealer fix or gate hold and retrofit on site. The last one involved an 800 hour engine test, the engines passed the test and production was released (based on product integrity and application testing) the 800 hours simulated the serviceable life of the engine.
Some suppliers have not been so lucky, just google the Firestone versus Ford issue, where people were alleged to have died due to tyre failure. Vehicles were recalled and the tyre plant was closed due to the issue. I still do not think it is settled.
I can only assume that on review of the facts they consider the product safe, and also the mode of failure as only an annoyance to the customer.
recall check websites below to see what safety recalls and investigations have been recorded on your model year: