Interesting point, but.. At the time this was happening, it was not known whether or not the police wanted the gear. After Swain suggested to Brown that the equipment be given away and Brown said it had to be held for police inspection - it would be even more suspicious if Swain demanded the gear and then Brown refused on the same grounds. That would be like - well, if you won't get rid of it - I will. Swain wasn't ready to go that far and I think it would have been even worse for him if he had. Take the equipment, get rid of it yourself, even after someone tells you it has to be held for the police. It would have been far better for Swain to persuade Brown to do it.
It was ruled "accident" until proven otherwise, and they were talking "inquest" for quite some time and the police never officially closed the case. That is why Brown held onto the equipment all those years.
Source: April 2006 -
Rhode Island news | projo.com | The Providence Journal
"..While Tortola police say the case of Shelley Tyre's death was never officially closed, their request for documents is one of the first active steps they've taken.."
In addition Brown was following protocol, whereas Swain did not appear to be interested in following that protocol:
"..Following divemaster protocols, Brown said, he took all of Shelley Tyre's scuba equipment -- including her air tank -- and locked it in his office to await inspection by Tortola police. But two days after Tyre died, David Swain came to visit his shop.."