It's pathetic, imo, that companies like twitter and facebook pretend to take a principled stand, when they chose not to take that same stand EARLIER when it was riskier to do so (but made a lot more sense).

What happened here is opportunistic, not principled. There were plenty of other cases i can think of where companies did take riskier stands after evaluating pros and cons (cloudflare springs to mind). Those you can call principled.

By waiting for the time when it was a "good PR move" to show support for BLM, they are signaling that they don't actually support BLM (if they did, they would've shown their support before), they actually just support having a good public image.

Basically, they are just full of shit and are willing to hop on whatever bandwagon makes them look the best.