It's not illegal - but it's also not helpful, or constructive, and nor is it desirable if you're trying to build a helpful community.
There are other ways of getting your point across, without resorting to insults such as "piece of s*it".
In Chinese, we might call this 家教 - or "upbringing". If you have no decorum, or class - it suggests that your upbringing was poor, or that your parents failed to raise you well.
We often forget that there's another person at the other end of the phone, or at the other end of our comment. In this case, a real-life person who put a lot of work into a product. You don't have to love it - but if you are going to criticise it, at least don't be a jerk, and make some good-faith efforts to work with them through the issue.
It's not exactly like the developer didn't try to help.
It is desirable if you're trying to build a helpful community where customers help other customers differentiate between good products and products that sometimes don't work on arrival or have software reliability problems; a community that recommends to avoid certain companies is a helpful community to have for a buyer.
It might be chinese culture to avoid criticize things directly, but it's also part of the chinese culture to give backhanded compliments like "5 out of 5 stars, when it works, which is never." I don't think the 笑里藏刀 (literally translated as "dagger hidden in the smile") aspect of chinese culture claims any moral high ground over the western straight shooting f* you.
I'm chinese too.
In their first and only reply they perma banned the customers device ID. How are they helping?
It's impressive that you're able to extrapolate the questioners life up until this point based on three words.
If the product in question is a shitty product, there's no problem describing it as such.
A more amusing instance of this outlook at work: http://www.cracked.com/personal-experiences-1940-china-one-p...
No, you don't get it. From a business perspective expecting good behavior from customers is just plain stupid. It's not customers' fault if they don't understand something and don't take time and effort to deal with your product and just call it shit, they are human after all. You can't defeat human nature. But you can try to understand it and at least be prepared to deal with it and minimize a negative impact on your business.
Customers shouldn't be absolved of having a minimum level of decorum, but that minimum is at the level of (a) personal insults at specific people ("Bob in customer support is a piece of shit" wouldn't be ok but "your customer support system is a piece of shit" is fine) or (b) libel, as decided by the legal system.
For direct criticism of a product or service literally anything that I imagine would be above that minimum level of decorum. Insulting people is not ok, but insulting products definitely is.