Picking up a Clearance Airborne

I was departing Harford Co MD airport yesterday (0W3) enroute to
Chesterfield MD (FCI). 0W3 is a small non-towered field with no ground
communications to ATC or FSS. In trying to pick up my clearance on the
ground, I attempted to contact Leesburg FSS on the 1-800-WX-BRIEF. Due to
call volume, I was routed to Altoona FSS, which did not have access to my
flight plan and said that I needed to call Leesburg directly. So I called
the direct Leesburg number, and after staying on hold for 15 minutes
(meanwhile my cellphone battery charge is ticking away) I decided to hang
up, and get my clearance airborne.

The weather was VFR, albeit a bit hazy. Even though 0W3 is just north of
the ADIZ, I really wanted to get my clearance prior to departure just
because my route would eventually take me into the ADIZ.

I ultimately decided that I would depart VFR to the north, and circle if
necessary, until I was cleared enroute.

After taking off, I contacted the departure freq on the approach chart for
0W3. The controller chewed me out for departing VFR, made a comment that it
was unsafe, and sent me to another frequency. I tried the new freq, and
this controller chewed me out even further, refused to get my clearance, and
told me I should have gotten it on the ground. Explaining to my passenger
(1st time flying with me) that the ground was a better place to deal with
this than the air, I returned to the field to start making phone calls.

I finally got the number for Potomac approach, got my clearance. I was told
to call back right before I was airborne. (Is a cell phone now a require
piece of comm equipment in an IFR aircraft?)

I know the controllers were busy at that moment and that's why refused my
clearance. Why didn't they just issue a sqawk code with the instructions
"remain vfr"? Does it have to do with the nearby ADIZ? I guess my
confusion is this: I've picked up clearances in the air on numerous
occasions. I thought it made it easier for them because they did not have
to block out airspace for my departure. I've even been denied a clearance
through an RCO on the ground at a Charlotte area airport and asked to pick
it up airborne. While I didn't appreciate the on-air chew out from the
controller, I am not really upset, I just want to know how I should know
these things ahead of time. I haven't been able to find guidance from the
AIM or 7110.65, NOTAMS or otherwise.

Brad