What can they force those witnesses to do?
Convince all of them to censor some specific transactions?
20 years of jail
Even if they manage to convince 7 witnesses to do something that specific, all of those 7 witnesses will be replaced with hard-fork.
good luck finding people who want to spend the rest of their days in prison
With that Liberty Reserve example, you are missing the key point, they got busted because of money laundering, the same way how exchanges that doesn't require KYC will get busted with money laundering (or no bank will not accept them if they haven't done it). Byteball witnesses have nothing to do with fiat currencies and bots that accept credit cards also do KYC.