It's not really for you or anyone for that matter to decide whether it should be paid or free. That call is down to the people that poured their time, blood sweat and tears into developing the add-on.
I don't know of any forum company that makes their official add-ons free (if they do it's a limited edition). For what you get the sale price of the RM is worth every penny but I'm not debating the pros or trying to change your mindset. My point is that it's up to the people who put their time and effort in bringing a product to the table, they price it accordingly and then the choice is yours whether you either want to purchase it or not.
Any add-on purchase may be canceled in the future, and the $15 fee will be removed from needing to be paid. However, you'll have to re-purchase the add-on at a future date if you'd like access again (at whatever price it is at that point in time).
Well aren't you just a delight. The RM is on sale for 24 hours from 12am GMT. Information on the internet is not always correct, maybe you should read it from official sources such as the XenForo homepage. I totally 100% disagree with you about the RM being free of charge. I am a developer myself and these people deserve to get paid for their work. A RM is a add-on and should never-ever be included as a the core package - it's just not something that every xenforo forum is going to use/utilise.