Thank you for bringing this to our attention @agmoore.
I was a blood donor for many years and donated plasma twice. Here in South Africa blood and plasma donation is purely voluntary and our blood clinics are always cheerful places.

Is it ethical to pay plasma 'donors'?
No!
Why are they still called donors when they're selling something?

I absolutely agree with this statement:

Because commercial plasma donation centres are located in areas of greater poverty, vulnerable populations are made even more vulnerable concerning their health safety..."

Frequent plasma donors need to be healthy. The temptation would be to do it for lengthy periods. It for sure will have a negative effect on their health.
Blood centres are supposed to check, but, they are private companies in the business of generating income. There's always someone who'll be willing to turn a blind eye and fiddle the records.
Further, it's a lengthy process, at least one and a half hours once the process starts.
They'll not be able to 'slip away' from work, will have to do it after hours when it's family time.
That's if they are working! An easy way to make money, less time to find employment.
The extra money will be to their own detriment and cost them in the end!