That's not right at all. Loads of stuff is inadmissible as evidence but not illegal. For example, in many countries phone intercept is inadmissible. For example, in The Wire, they might hear a drug deal being arranged on the phones, then arrange to be on patrol with a drug dog and find the stash. The phone recording would be inadmissible (even though it would help the prosecution), the evidence used to prosecute would have to be paralleled.