Her heirs would have disagreed. Consider the case where your mother owns a house and 'sells' it for $500 just before dying.
Also, I expect people buying such rights are generally well-off, and the people selling it to be relatively poor. After all, the buyer must be able to pay money on a house they cannot live in for an indeterminate period, and the seller typically chooses this construct to be ensured of both a roof over their head and money to live from for one's entire life, no matter how long that is.
The odds in this lottery get better every month for the buyer, so, assuming that the buyer can afford to buy into this, they should have no reason to want to get out of this.
On the other hand, having someone pay tons of money and then die without anything to show for it, I do see, even if he "willingly signed into it".
There's also a threshold over which it should just be handed over. That such a threshold wasn't in the contract doesn't make it any less odious.