Because not attesting is against the network's fork-choice rules. If that censoring majority chooses to avoid said block, because it contains transactions that should be censored, they will perform an illegal re-org around the block. Sure, it won't be "illegal" as a majority of the network follows said re-org.

It's an extremely nuanced topic, and it was extensively discussed in the last core-developer call on Thursday. Every single operator has been warned that going against the fork-choice rules could, and ultimately will, result in social mitigations against spec-deviating behavior.

This could be set of socially executed slashings of validators that don't respect the fork-choice rules, or an honest-minority executed hard-fork that effectively causes misbehaving validators to bleed until they start respecting the protocol's rules.

I could be wrong on multiple counts, especially regarding re-orgs, but this is how I've understood the issue.