Why would Boeing be under review if crew-error is determined to be the cause? The captain on Lion air's flight 610, re-trimmed the aircraft through 21 MCAS activations. The first officer, who was given control of the aircraft, got 5 more. Although it is speculated that neither pilot executed the required stabilizer trim procedure, the report does not specifically state whether or not that procedure was actually run. Source 

The captain on Ethiopia Airlines flight 302, tried engaging the auto-pilot, on the faulty side as well, and despite that being the first step for the procedure, to disengage, he tried multiple times to do otherwise, then skipped over the second step, which was to disengage auto throttles, this allowed their airspeed to build up around 390. I’m sure this didn’t help the nose down force, but ultimately, they re-engaged the system which allowed MCAS to drive the nose down further. Boeing's CEO stated these failures to follow procedures is what led to the crash. Source

The question, then, is this: if the crashes each resulted from pilot error, why is Boeing under so much scrutiny?
In aviation (and other safety critical areas) it's not enough to just look at the first cause of an accident, you have to look at why that mistake was made, and why that thing happened, as far back up the chain as you can. Then you try to address all the problems.

So, when a pilot makes a mistake, it's tempting to say "the pilots was an idiot. Case closed." However, then you've got to wonder why the company employed idiots, and how many  others are flying. That implies that there's something wrong with the pilot licencing and testing rules, and that has huge implications.

If we assume that these pilots weren't idiots, we have to ask why they made a mistake, because other good pilots could make the same mistake. Anyone who has played a computer game knows that mistakes are inevitable when you're under enough stress, just like a well built bridge will still break if it's overloaded.

You could conclude that they hadn't been trained well enough, which raises the question "Why?". Apparently the pilots did all the required training, so maybe the training was at fault. But it's not enough to just improve that piece of training, you've got to ask why again. Was it not checked? What else hasn't been checked properly? How do we know that any future training will be properly checked? Also, what incentive did the person writing the training have to miss things out, and does it apply elsewhere?

If you don't find and fix the root of a problem, it will continue to grow until it pops out somewhere else and causes another tragedy.