It's not that you can't complain - we see plenty of evidence that you can!
It's more that, if you're not financially supporting the devs, why would they care or listen to you? Why would they even know that your opinion exists?
When you get a product for free, of course you can still complain about the quality, or functionality.
But it was free, so either bin it & move on, or ask yourself "what did I expect, for free?"
But if you voice your complaint in a constructive way, people are likely to accept it, at least they can understand why you hold that opinion.
If you do it in a way that just whinges & adds nothing of substance, people are less likely to accept it.