Generally a reverse burden of proof would only apply if -
the legislation specified as much (e.g. it shall be a defence if..., or the legislation creates a presumption),
the legislation would be unenforceable without a reverse burden (e.g. insurance),
or the authority have already proven their case to the applicable standard.
Presumably, the authority are of the opinion that evidence that a vehicle exited the scheme (at a different point to that at which it entered it) within a certain time period is sufficient evidence that it was not for access.