the essential reason of this kind of persistence is that it has to survive the explicit deletion of the cookie by the user.

Browser offer the user the possibility to remove cookies (manually or delete all), and this is because users want privacy.

This clever library manages to exploit browser features to go around this and store some identification information persistently against the will of the user

If the user doesn't want to delete or disable cookies, then you can use normal cookies for the purpose of legitimate "remember me" functionalities.

However I can understand that not all users know what cookies are, and there are many people who might have cookies disabled by default (sysadmin choice in a company) or somebody told them to do that.

So, you could reason this way:

"There is no point disabling the cookies anymore, since anybody could employ this trick to circumvent it. There are people which disable cookies because an obsolete 'security policies' which is not anymore secure. I want to make a webapp that works for everybody. It requires cookies. People are paranoid but employ obsolete security policies which don't protect them anymore. I can exploit the same trick to circumvent their default security policy for morally good reasons"

Your exploiting a security hole in my browser and overriding my explicit wishes to benefit your company is no more ethical than my exploiting a security hole in your website and "fixing" your database.

How many software include tricks to get around firewalls by punching holes (http://www.h-online.com/security/features/How-Skype-Co-get-r...). Is this unethical because it circumvents an explicit user wish? Do people even know that they have a firewall, or know what a firewall is, or do people even have control on the firewall settings (at work for example)?

Of course they want to run e.g. skype, who doesn't, right?! I know that there is something arbitrary in all that, that's the point.

I didn't say it was ethical to circumvent the user wishes. I said that some people might reason in such a way that it makes them feel morally excused for exploiting something which is perceived as an unethical technique in order to perform a licit goal.

The main points behind this mind setting are:

(here "you" are the application devel, not the evil guy, of course)

* point out the user de facto doesn't have control on his privacy settings by disabling the cookies, since the Bad Guys (TM) already have a hack to go around it.

* point out that the user is not even conscious of what privacy and security risks are, and often run a browser preconfigured by the sysadmin, nephew, whatever, which might decide to conservatively block cookies "because they are bad".

* you are not exploiting the cookies with the purpose to invade user privacy. You are just building an application X (see grand parent question) which exploits the same hack to get around the 'default paranoid settings'.

* you feel stupid to limit your application functionality just to obey some obviously bugged rule. It would be like skype saying "oh, there is a firewall, I know how to get around it, but I won't because it's unethical since people have the right to setup a firewall according to their wishes".

(of course these points are valid once this technique becomes mainstream, and all tracking sites employ it)

I'm not saying that behaving this way is ethical or not or less unethical. I'm just supposing that there might be some uses of this technique which are not directly intended to trace the identity of a user for malicious reasons (marketing etc) but for providing some functionality to the average user of a particular product (who asks for it).

People might be pissed out because some features don't work. They don't care why. Application providers are also pissed off when half of their users cannot use a given feature because some sysadmin/security software/nephew hacker decided to impose some restriction (settings, firewall rules etc), even if there are valid reasons for the restriction (settings, firewall, etc) to be be there.

What about combatting trolls?

What about shutting down websites I don't like?

They are not impinging on any right of yours.

what about free speech?

Free speech does not mean you are allowed to go anywhere and say anything.

The deletion isn't always explicit. Several consumer "web security" applications delete cookies automatically and the user may not be aware.