While there might be issues of regulatory capture (ie too much influence of the regulatee (<- not a word, but it should be...) on the regulator)), my personal opinion is that there were two consistent courses of action:
1. Ground the MAX after the reasons for the Lion Air crash had became clear, as it's not airworthy.
2. Do not ground the MAX, as it's sufficiently airworthy.
This new crash had not provided enough information initially to conclude otherwise.
However, the FAA is sort of arguing that there is now enough new information from the second crash to support the decision to ground it.
This is the answer. The FAA considered the Max 8 after the Lion Air crash and determined that it was safe to fly, even taking into account the cause of the Lion Air crash.
If that was determination was done correctly, then there is no reason to ground every plane of that model, just because another one crashes shortly thereafter.
The FAA obviously is inclined to believe that they did that determination correctly. Of course, they could be wrong.
It will be very interesting to see how the Ethiopian investigation turns out. If the cause was the same as Lion Air, then the FAA will come under tremendous pressure to reconsider what constitutes "safe to fly".
(IMO the industry in general will come under hard questioning about the role of software in compensating for aerodynamic issues. Yeah the military does it, but we have different standards for airlines.)
But if it turns out that some other factor caused the crash, unrelated to the Lion Air cause, then the FAA will come off looking insensitive but ultimately justified in their determination.
1. Ground the MAX after the reasons for the Lion Air crash had became clear, as it's not airworthy.
2. Do not ground the MAX, as it's sufficiently airworthy.
This new crash had not provided enough information initially to conclude otherwise.
However, the FAA is sort of arguing that there is now enough new information from the second crash to support the decision to ground it.