The point of the free trial is to get you to try the service and (hopefully for them) decide you like it enough to buy it. Someone that has already tried it free (or even had a paid subscription already) and chosen to not continue the service has obviously made up their mind already.
Amazon Prime is the same way. I was signed in to my account while browsing the site, saw a huge banner advertising the free trial of Prime (even though I'd already gotten a month free a few years ago) and figured I'd go for the free trial again as I was ordering something. Next thing I know, my card was charged for a full subscription price that same day. Turns out, free trial was only for people that hadn't had it before.