This does sound a shocking incident, but unfortunately it doesn't surprise me that something like this has happened. I'm sure it won't be the first or last time...
Many ambulifts in use are as old as the hills and have very little in the way of safety features. Many aren't even purpose built: I've seen an adapted forklift truck used for example.
With ground handling contracts/budgets being slashed to a minimum there is little money left for investing in new, very expensive (this is a niche market) equipment. Sadly what is in use just has to do the job.
With the lack of failsafe features in place it falls to the operator(s) to be extremely vigilant and competent at using the equipment. This comes down to training... another area where budgets are cut to the bone.
Ambulifts are inherently unsafe in my opinion, however the risk always has to be balanced by the benefit. The only other option is manually carrying WCHS/WCHC pax up the stairs in the aisle chair, hardly a safe or desirable practice.