Yes, as a previous vendor systems (not offered now), I tracked that my subscribers made thousands and thousands of real dollars.
While your attitude is understandable, these sorts of arguments "could make more money by trading it then selling" don't hold up. Now, in my case I did take my systems private because I wasn't making enough from the subscriptions. But, I might reintroduce them in the future.
The first thing to understand is that there are no absolutes in trading. There is always risk. So, there are no guarantees. A trader can make money every day of the month and then have a bad day and lose it all in one day.
So, once you understand that there are no "risk free" profits in trading then you can understand the first reason a trader might sell a profitable system: the reason is that no matter how profitable a trading system is, it requires both capital and the taking of risk to trade it. System developers may lack either capital or not want to take the risk.
More over, a good systematic strategy might return say 50% or even 100% per year. Systems that become popular are known to make their developers, 10k or even 15k per month. A small strategy developer starting with a 10k risk capital stands to only make 10k trading his system over the year but could make 180k in offering subscriptions.
So, you can see clearly for the small trader that selling the signals can work out better. But, even for a moderately capitalized trader, let's say this trader has 100k but only wants to allocate 25% of his capital to this system, which gives 25k profit over 1 year versus again 180k profit, and even with 100% allocation, selling the signals can produce 180k vs 100k. And now factor into that logic, that there is nothing preventing the trader from both trading and selling the signals you can start to understand why someone would sell the signals.
So, yes it is no surprise as to why a successful systems and signals are sold. Generally, I'd be less weary of signals sold then outright systems. In general, it is best to see third party auditing unless you know the developer.