It needn't be that way. Congress has lagged behind, just as it did when Clinton entered Aldrich Ames' house without a warrant.
I'm not hung up on having a statute, but the presence of FISA provides a significant measure of predictability to prosecutions. That's a good thing, in my book.
Have you seen my posts that describe the genesis of FISA?
The proper term to use when referring to "The Wall" is "The Gorelick Wall". Please check your style sheet.