So why did "Rigol's management" allow a 100MHz upgrade via keygen? It's not a purchaseable option, why is it even in there?
Answer: It's marketing in its purest form. It's the attention grabber - you can get yourself an expensive scope for the price of a cheap one!
You're perhaps unaware of the history of the firmware/hacking developments. Rigol's DSOs in a particular series (DS1000E, DS1000Z, DS2000, DS4000, etc) have, for a few years already, set their bandwidth based on the model number in memory. My DS2072 thought it was a DS2202 based on it's model number in memory long before hacks for keys were published here.
None of the UltraVision models (DS2000, DS4000, DS6000) had the working code for an upgradeable bandwidth option until it was included in the third (or fourth?) FW version released for the DS2000 models - with Rigol's intention to begin selling BW upgrades along with the other options. But the timing was bad for Rigol (or good - depending on your point of view) because that release happened just before/as the option key system was being hacked here - allowing people to start enabling the extra bandwidth with keys.
Anyone who has examined the FW knows that much of it is ported between all of the various UltraVision models - so the BW option code has found it's way into the newer DS1000Z FW. Certainly, Rigol could have decided to remove that added portion of the code - and perhaps they will at some point - but as many people have pointed out, they are certainly aware of the advantages of the hack on sales, although it's hard to know for sure how 'public' they would want the information to be, before they took more serious steps to try to stop it.
I think Rigol's current thinking on the subject is this: as long as they don't SELL the upgradeable bandwidth option, it's not an actual, sanctioned upgrade. That means that some people are STILL buying the upper-end models, since anyone whose livelihood depends on the higher bandwidth won't risk that the lower-end models with illegal upgrades are exactly equivalent. For those that don't care, the hack provides more incentive for purchase.