They are incentivized with higher fees. 

So what can a billionaire do to the Bitcoin network? Buy half the mining power. Can they censor a transaction? No. The power of the network is that only a single miner needs to create a valid block for a transaction to not be censored. Can they reverse a transaction? No. Can they "spam"  the network with 1 sat dickbutts? Sure. So the other miners who instead build blocks with high-fee transactions will see their revenue skyrocket as competition for block space increases. This incentivizes new miners to enter the space and capture those high fees. Meanwhile the attacker is wasting money AND increasing revenue to "normal" miners. 

With Drivechains, miners are not required to merge mine, and they aren't obligated to any particular chain. They can stop mining or a competitor could come in and immediately take over 90 percent of the hashing. It's possible that the incentives will be for each mining pool to stick to one drivechain, allowing each have a virtual monopoly. Basically, the point of failure has become smaller and cheaper. No single government regulates *all* Bitcoin mining, but it would be trivial to manipulate a single pool. 

I think the broad idea is that because drivechains look so kind-of nebulous and ephemeral in how they "attach" and"detach" from various miners at whim, people wouldn't be afraid of being rugged so much as the whole drivechain just disappearing or potentially being dominated by a censorship-friendly pool. The end result is no one wants to use it FOR MONEY, so all drivechains and up getting used for is scams and spam. 

I think miner centralization is a very serious issue, but rewarding them in this way doesn't stop centralization, and doesn't scale monetary use of Bitcoin.