There have been miners that rejected transactions with OP_RETURN because its unnecessary spam. Virtually every node and miner limits OP_RETURN outputs by size.
The amount of data relayed or allowed by default is not very large, one of the reasons for this is reducing drama about "illegal data".
We've come up with even stronger techniques that could be used to prevent the inclusion of arbitrary data in transactions, if it were ever required.
But the idea the some invisible data squirreled away in an inaccessible location would be legally dubious seems impractical at least for small amounts of data: consider, any text could secretly convey a small amount of data-- so a hard-line adherence to that theory of liability would make it impractical to ever publish any text provided by a third party. I think there would be a creditable argument that in the US any law which attempted to do so would be invalid on its face by outright inhibiting the press.