The problem isn't the jailbreak - it's that Apple has no means in their sandbox to tell whether or not someone has paid for a product.
I'm an Apple Dev, and I have an app where I did something similar to this. The reason? As with most app devs, there are analytics on my apps. Those analytics tell me generic things how many copies of the app are installed.
It doesn't take a rocket scientist to compare the number of installed apps to the number of purchased copies. Apple allows you to install a purchased app on up to five devices - so at the very HIGHEST there should be a 5:1 coorelation of purchases to installs. In reality, this is actually just below 2:1 because most people don't have that many iOS devices
One of the apps I released, on the first weekend, had about 100 purchases - it had maybe 5-10 per day over the next week, and then another couple hundred on the next weekend - the trend continued...
By the end of the third weekend - there were over 50,000 copies installed. This is absolutely rediculous. Unfortunately, as a dev - we have no way of telling whether or not the app is actually purchased. A quick google search on my app instantly found it out in the Cydia app store, and a couple other popular "pirate" sites - cracked, and ready to distribute.
The moral of the story - the app was 0.99 - buy it. Don't pirate it. Someone like me (an indie Dev) would have had several thousand dollars to support future dev efforts, instead - we get a couple hundred dollars and a useful app that ends up on pirate sites and we give our time away.
Our options - take measures to prevent usage on jailbroken phones (sure, some people jailbreak for legitimate reasons, but the majority - based on the numbers I have seen, do NOT) - or, just accept that all of your effort will end up free to the masses.
At this point, the only thing I can do is release a good free app, and hope to make some money on ad revenue... But then, the jailbreaking wizards ruin that too by releasing apps that run in the background to block advertisements - so again - I lose. I'm expected to work for free.
That being said - I do not enjoy working for free - would you? There is definately a place for jailbreaking - unfortunately, for someone who takes pride in their products and hopes to release an app that "takes off" and includes monetization (either paid or ad supported) to support their livelihood - I tend to take a dim view of the reality that a "place for jailbreaking" exists in it's current state and given the tools that we are provided for being "legit" developers.
Just my .02 from the peanut gallery.