Many companies use the word 'free' in a way which most people would regard as misleading. Try searching for Windows software using such terms as 'free DTP program' or 'free video editor' and you'll quickly see what I mean. The majority of such programs turn out to be only trial software, limited by time, by what the trial versions can actually do or (for example) by sticking a dirty great watermark on your publications or videos. (I've been looking at some 'free' Windows programs lately, where a bit of digging into their websites showed that the cheapest of them actually costs $120, with others costing up to $400 before you can use them properly)
'Calm' seems to work on a similar basis, where 'free' means that you don't have to pay to download the software, or to create an account, or to use the app for the first 7 days. Beyond that though you have to pay:
https://support.calm.com/hc/en-us/articles/115002582768-I-just-downloaded-this-app-Am-I-going-to-be-charged-
Companies aren't going to spend millions of pounds on TV advertising without expecting something back!