Because he is an "evangelist", of sorts. Spong is the foremost representative of that branch of revisionist theology that emphasizes the forgiveness aspects of Christianity, and completely ignore those inconvenient parts about judgement.
It's not just an Episcopal thing, of course: you see the same pernicious agenda in all of the mainstream denominations (note that he spoke in a Methodist church in the article). Theirs is an agenda which is best spread within large national churches: groups can infiltrate the national bodies and work their way down. In this sense, the situation of the mainline protestant churches is not terribly different from that of the Catholic Church before Luther and Calvin.
Beyond that, Spong loves the limelight, and his present status cannot be maintained without the attention-grabbing act of defiling Christianity within the confines of Christian churches.
To its eternal shame, the national Episcopal Church, and many of its bishops, have refused to address Spong's apostasy -- which, as you've noted, is all in the service of "anything goes." Part of it is cowardice. But in many cases, the bishops agree with much of what he says.
Speaking as a member of a vibrant Episcopal parish, I can state that all is not bad in the ECUSA, it just looks that way sometimes. IMHO, for beauty and content you just can't beat the traditional Anglican liturgy, hymnody, and theology.