I would not consider that a scam, but a business tactic.
It can work in your favor or not. It will be for your favor if you want to continue to use the service, then it will go automatically, save you the trouble of renewing later. If you don't want to renew and don't remember to cancel, then it will be against you because you will be automatically charged.
There are other sources for free listings - your title company, your county, your favorite Realtor, etc. However, RealtyTrac or other sites does come in handy if you know what you want to use it for. For $50, it could be a wise investment, but all up to you.
The other similar things I can see are buying something without paying interest for a year - if you don't remember to pay it off by a certain time, you'd be charged full interest (and very expensive rate) at the rear.
A consumer should also be responsible for their own action (meaning getting free access and then cancel if they don't like it), then no, it's not a scam.
if a consumer depends on others to remind them, then they should not use this service anyway.