A principle fundamental to science, and to scientists, is that 'extraordinary claims require extraordinary evidence', and yes, if it is a third party making the claims against somebody, the onus is upon them to prove those claims true, not for the party the claims are made against to prove otherwise, so as to disallow a 'win by default', which in logic, is not a valid outcome.
For example, if you were to accuse me of being a paedophile (and I'm not for a minute suggesting you have or are), it would rest upon YOU to prove that *I* had abused the child, not my responsibility to prove that I did not. You would be the one who must present the evidence, and not I the one who had to refute all hypothetical possible sources of that which could potentially be seen as evidence by SOME party, by however slim a margin. The latter equates to a herculean task, and one both impossible and impractical, for it is not possible to prove a negative. One can of course prove a person IS something or DID something, but proving that same person ISN'T that thing, or that they did NOT do a thing is quite different and logically impossible. And what is more, demanding the accused prove their innocence rather than demanding the accuser prove the guilt of the accused is neither fair nor reasonable. That gives way too much (false and undeserved) of a shadowy, nasty sort of pseudo-credibility to accusers at the expense of the party facing the accusation.