It's not about being free or paid, but it's all about being honest to the users.I don't have a problem to sign up or pay for the service. But I need to know before I start using it.
It's like I install a brand new text editor software, spent some time to write a story, and when click on the "save" option, it asking for the payment - that's dishonest and shows that there can be more such dishonest behaviors in the future. But if the same software asked me for a payment upfront, that's totally understandable. Or, if it said upfront that "this is a limited demo, you can save documents up to 150 words, and if you have to save larger documents, you need to pay", that's being very straightforward to the users.