Same reasons as the context of this photo (1). One party would like to advance some geopolitical interest, another party is willing to do it if they are paid and supported as such. No different than any other business deal.

1. https://upload.wikimedia.org/wikipedia/commons/b/b8/Reagan_s...

Is there any evidence of this? Why would pirates not advance this claim?

I think it's much more likely it's just easy money and is relatively cheap to pull off.