A lot of posters here with no idea about how the duchys work.
Firstly, they are not run by William and Charles, they have boards who make the decisions collectively and in accordance with the rules set down by parliament as to how they operate.
Under these rules, they are required to rent out their land at commercial rates. Parliament has passed laws requiring that, and Treasury policies it. From HMT’s Memorandum about them approving the Duchy’s transactions:
When assessing proposed large property transactions under s11 of the Act,
the Treasury seeks evidence that the terms are commercial. Helpful indicators include:
• for sales, competition among potential purchasers and at settlement prices in
line with estate agents’ guide prices;
• for investment and development projects, actual or expected returns at market
level
Also, it is not good governance for charities to operate on a non-commercial basis - that becomes a large financial stability risk for them (what if they need to move and cannot find a new property for free?). So if they enter into leases, the charity commission expects them to be at appropriate commercial rates.
As for theft - most of Charles’ assets come from Blanche of Lancaster marrying John of Gaunt in 1359. She was a considerable heiress as the sole surviving child of her father the Earl of Lancaster. Their son Henry Bolingbroke, later Henry IV, inherited them in turn from his mother, and they had been part of the sovereign’s estate ever since. So no, not stolen.