The Catch-22 of this situation is that SeaWorld/Cedar Fair are arguing that Theme Parks are distinct from the other categories they're lumped in with, and Northam is essentially agreeing with them, which is why he's including him with that group to restrict them from opening. Northam is focused on the sheer number of people who would be present and the higher likelihood of those people being out of state (which is a problem for all tourism operations), and the scale of handling that level of touch points and any kind of contact tracing from a potential infection (which is, for the reasons the parks are distinct, more significant than beaches, for example, which have fewer choke points and shared surfaces). Whereas SeaWorld is focused on the sheer scale of the park and the elaborate safety measures they're proposing which, imagining a situation where someone is infected with the virus, the combination of a beyond-state approved mask policy and proper social distancing should limit the potential risk posed by such an outbreak. Both sides agree that BGW is a distinct case, but they disagree on whether that means they are the SAFEST place to be or the most DANGEROUS place.
The best way I'd summarize it is that if you frame this through the lens of personal responsibility, BGW—provided safety guidelines are enforced, which is clearly no guarantee—is probably safer than other forms of tourism or amusement, given the policies that are in place, the presence of employees tasked with enforcing those policies, and the way the park's regional focus and strong membership base create a stronger investment in the park that could push compliance compared to tourist-focused parks in Florida. But the fact is that the state isn't framing this through the lens of personal responsibility: they're framing it as a macro-level public health problem the second it starts getting into thousands of people sharing space in a space where historically viruses spread (if you're never gotten sick after visiting a theme park, you're a unicorn).
It's a weird scenario where I feel like I—as a person who is a member, lives within 45 minutes, lives alone, can self-isolate without compromising my employment, and is not in an at-risk group—feel as though I would be willing to visit the park with the understanding I could leave if I felt unsafe without consequence, but at the same time I think the Governor's Office isn't wrong to feel like a theme park is a nexus point for the type of community spread that is spiraling out of control in other states. It's possible clear data emerges in the next couple of weeks about whether any cases can be traced back to major parks in Florida/Texas/etc. that might tip the scales on their understanding of the effectiveness of their safety protocols, but today revealed a clear gulf in how they're conceiving of the parks that is less a grand political conspiracy theory and more just different visions for how the parks fit into a public health framework.