It isn’t unethical in principle if it’s self-contained software of the sort @ozfiddler mentioned. But let’s give you a scenario that @anon66243189 has touched on, and you’ll see how potentially unethical a trialware plugin could be.
This plugin stores user data. The website owner decides not to keep the plugin at the end of the trial, and now that data is inaccessible (and, for all we know, has actually been lost or destroyed altogether, though it could equally well be sitting in a database somewhere). Now a user approaches the website owner and asks — in accordance with the law in many jurisdictions, including where you are, @Paul — for details of what personal data the website has about them. But the website owner can’t answer that, because they no longer have access to the plugin. They could point the user to the plugin developer, but it’s quite possible that there is no way for that developer to identify the relevant account.
So now we have a website that’s in breach of the law because of the design of the plugin. Think the user won’t do anything about it? Think again! Many of those queries are triggered by credit reports from Experian and the like, and so the user wants the issue addressed. So they complain to the jurisdiction’s Data Protection Registrar, and an investigation ensues. Has this happened? Oh, yes! The person who took over the Display Widgets plugin had this happen.
Do you think the investigating agency would not bother to investigate the place where the website owner got the plugin from? Certainly, if I were one of the Directors of the Initiative or a plugin reviewer or moderator, I would not be feeling too comfortable.
So, I agree with this: