Solja, you seem to have missed the point. Insurance is based on risk, and the premiums and clauses that the insurance company will underwrite are based on their assessment of the size of the risk.
Underwriting is a science to itself, but on highly simplistic terms it might go like this.
If Mills has a career ending injury, at best his payout might be (guessing) $5 million. Lets say there is a 0.5% chance of that happenning, his insurance premium might be $25,000 plus some fat for the insurance company.
Bogut is a different story. His payout could be worth $100,000,000, and his risk of injury, given his history, could be, say, 5%. The insurance company would then be wanting an insurance premium of $5,000,000, or alternatively would be seeking to exclude the exacerbation or repeat of an existing injury from the policy cover. In these circumstances insurance is no longer commercially viable and most insurance companies (even if they were to delve into this specialised area) would not touch it with a barge pole.
The NBA, as a collective, can approach an insurance conglomerate and obtain a competitive price, because the risk of injury will be spread across a whole league of players (which is reasonably predictable) which makes the task of underwriting far simpler.
The above figures are probably somewhat inflated, but you get the idea. The same basic assessment applies to the premium on your house based on your risk of flood, and explains why some houses in Brisbane are simply uninsurable.