This is politically motivated pure and simple. Most companies dump their boosters in the ocean and no one bats an eye. Unfortunately Musk should have foreseen not rolling the dice on the starlink mission when the Polaris got pushed back.
Any deviation from normal operation may result in a grounding. Groundings can be implemented by the operator, the manufacturer, or the authority having jurisdiction.
Groundings are typically caused by a request for an investigation or an emergency airworthiness directive mandating inspections or repairs.
Last week Boeing themselves grounded the 777-9 and it is a prototype that doesn't even carry passengers. When there was an uncontained engine failure on a United 777 all 777s with the Pratt & Whitney PW4000 engine were grounded. When there was an uncontained engine failure on a British Airways 777 all 777s with specific types of the GE90 engine were grounded.
When there was a door plug blowout all Boeing 737-9 MAXs were grounded.
I'm not just picking on Boeing, A320neos with the PW1100G were grounded.
And it's not just the US, though the FAA seems to be a little more aggressive. EASA grounds aircraft in Europe, Japan grounded an entire airline for weeks after a maintenance worker was found to have done repairs and inspections while drunk, and Qatar grounded an Airbus type due to chipping paint.
In this case a booster of the same type that is scheduled to carry humans into space had an engine malfunction during its landing and the FAA is waiting for SpaceX to determine its cause.
If a Delta A350 got done flying and was pulled onto an apron for storage prior to maintenance and it just "blew the fuck up", it is highly probable that the FAA would ground all A350s until it figured out what caused the explosion.
The Falcon 9 was grounded for several weeks last month. SpaceX performed an investigation said "liquid oxygen leak, we'll double-extra check it next time" and the FAA was like "roger dodger space cowboy" and flights resumed.
It's normal.
If you file a flight plan from New York to Sydney and have to ditch in San Francisco, the FAA isn’t out for you because someone else flew to Denver.
Put another way, this triggering not only your suspicions but rendering a conclusion around bias is a strong indicator that your calibration is off. (I’m saying this as someone with a huge bias in SpaceX’s favour.)