ACCESS EXCLUSIVE lock

Hi!

I have a problem with ACCESS EXCLUSIVE lock when I drop a reference in 
transaction. I have 2 tables: 
create table  a(id SERIAL primary key);
create table  b(id SERIAL primary key references a(id));

After that I have 2 processes: P1, P2
In P1:
begin;
ALTER TABLE b DROP CONSTRAINT  b_id_fkey;

In P2:
SELECT * FROM a;

And I'm waiting for the result, but I don't get until P1 finishes.
I know the DROP CONSTRAINT put an ACCESS EXCLUSIVE table LOCK into the 
TABLE a, and the SELECT is stopped by this LOCK in P2.
Note: I cannot commit the P1 earlier, because it's a very long 
transaction (more hours, data conversion transaction)
My question:  Why need this strict locking?

In my opinion there isn't exclusion between the DROP CONSTRAINT and the 
SELECT.

Thanks for your suggestions!
Regards,
Antal Attila