<<The other way might be a subtle warning that interfering in al-sadr's fate could be costly. >>
The point being that for thirty years the U.S. felt it could issue all the "subtle warnings" (some call them threats) it wanted to without actually submitting to the indignity of meeting with these ruffians face-to-face. The ruffians always said they were willing to discuss matters with the Great Satan but the G.S. did not deign to meet with them. Looks like (a) the ruffians weren't receptive in the slightest to the U.S. way it chose to deliver its "subtle warnings" and (b) the U.S. finally decided to get off its high horse, speak to the ruffians in the way they wished to be spoken to and has now decided, its threats having been exposed for the empty bombast that they are, it must "ask for" (translation, beg, plead, bargain with the Iranians) for something it desperately needs in Iraq but can't get on its own.
You can be sure the Iranians will not give their cooperation for nothing. It is not they who are coming to the U.S. to ask for anything. It is the U.S. which is coming to them. Always an inauspicious way to begin a negotiation. Had the U.S. extended its hand earlier, things might be very different.
<<ain't diplomacy grand?>>
For the U.S.A., diplomacy, not war, is the measure of last resort. In their case, it indicates the desperation and weakness of a once-mighty power. It's a well-deserved public humiliation, the more so since Iran is one of the world's premier violators of basic human rights, a country where homosexuals are publicly hung from cranes, religious dissidents such as the Bah'ai are raped and then tortured to death in prisons and where the state-employed murderers of journalists and intellectuals walk free in the streets. A country which the U.S., itself not exactly a torture-free zone, felt free to denounce and castigate at will for its shameful record of atrocities. And the U.S. now needs big favours from them and has to abandon its whole hypocritical act. Sweet.