They're charging for convenience and the ability to download a tutorial in pdf format, it's not like user experience is sabotaged for everyone. Assuming they don't get enough revenue from advertising, it's a decent monetization strategy.

They're not actually impeding anyone from viewing content, say for instance like Quora blurring posts or similar.

Nothing wrong with that, I wished people were more understanding of how companies sometimes need to do mildly unpleasant things in order to keep a product afloat so that everyone can enjoy it. Some of the things on Instructables are pretty awesome, especially a few years ago I would've had a very hard time in finding an equivalent for their various technology tutorials.