If you really think about it, exactly what evidence is unalterable? Photos can be altered. Text can be selectively edited. Identities can be faked.
On the otherhand, consider that EVERYONE who consents to load and use an instant messenger or send an email has already implied permission for their conversations to be recorded. After all, if you don’t IM, you can’t be accused of chatting inappropriately.
Who said the internet was fair?