You can understand the USDA claiming they don't have the resources to get involved in private testing for a disease they say isn't an issue here in the US anyway (a statement that can't be backed up with fact since we don't test).

But to spend what little resources they do have to sue a company to prevent them from testing - not only does it draw attention to their priorities, protecting business at the expense of public health, but it draws suspicion as to the true BSE infection level in US cattle.