I am not sure whether 'rent' is the right word, as it implies a monthly charge for the privilege of having the box, and there is no monthly charge if you do not want or need QMS. Sky themselves say they offer the box at no cost, and the customer is paying for the installation, as they need to send the engineer out to install it and change the LNB.
The reason they decided to go down that road is simple; customers who had (or still have) Sky+HD would often not be aware that they own their boxes (which were given to them for free at the start of the contract, they were only paying varying amounts for the installation, starting at £10) and when they were cancelling, they were asking Sky to come and take the box back or uninstall the satellite dish. Moreover, whenever their boxes stopped working for whatever reason, they were calling the company to ask for an engineer visit, but they were not happy with paying for the service call (even if it was their box that broke down) because they were saying it was Sky's box and Sky who provided the box to begin with, so they should provide the engineer as well, or else they would be cancelling. As a result, many customers were getting free engineer visits, and if their box was faulty, the engineer was replacing it for free, so in many cases the customers were getting a new box for free, a box that they would own. Not to mention that many customers were and are still cancelling every day, so their boxes are lost, sold, thrown away, etc.
As a result, instead of wasting money on free engineer visits and replacement customer-owned boxes, Sky decided to change things. They still own the Q, they are probably planning on using the returned Q boxes for parts or for refurbishments in the future, but they are also offering free engineer visits and warranty for life for these Q boxes (meaning that many customers can cancel their Sky protect, or take their Sky+HD boxes off the insurance policy, meaning they are saving money that way too). This essentially means that after paying for the initial installation, the customers do not need to pay for the boxes again (unless they cancel, rejoin, decide to take out extra boxes, etc.).