What's the difference between these T-SQL queries using OR?

I use Microsoft SQL Server 2008 (SP1, x64). I have two queries that do the same, or so I think, but they are have completely different query plans and performance.

Query 1:

SELECT c_pk
FROM table_c
WHERE c_b_id IN (SELECT b_id FROM table_b WHERE b_z = 1)
  OR  c_a_id IN (SELECT a_id FROM table_a WHERE a_z = 1)

Query 2:

SELECT c_pk
FROM table_c
LEFT JOIN (SELECT b_id FROM table_b WHERE b_z = 1) AS b ON c_b_id = b_id
LEFT JOIN (SELECT a_id FROM table_a WHERE a_z = 1) AS a ON c_a_id = a_id
WHERE b_id IS NOT NULL
  OR  a_id IS NOT NULL

Query 1 is fast as I would expect, whereas query 2 is very slow. The query plans look quite different.

I would like query 2 to be as fast as query 1. I have software that uses query 2, and I cannot change that into query 1. I can change the database.

Some questions:

• why are the query plans different?
• can I "teach" SQL Server somehow that query 2 is equal to query 1?

All tables have (clustered) primary keys and proper indexes on all columns:

CREATE TABLE table_a (
  a_pk   int NOT NULL PRIMARY KEY,
  a_id   int NOT NULL UNIQUE,
  a_z    int
)
GO
CREATE INDEX IX_table_a_z ON table_a (a_z)
GO

CREATE TABLE table_b (
  b_pk   int NOT NULL PRIMARY KEY,
  b_id   int NOT NULL UNIQUE,
  b_z    int
)
GO
CREATE INDEX IX_table_b_z ON table_b (b_z)
GO

CREATE TABLE table_c (
  c_pk   int NOT NULL PRIMARY KEY,
  c_a_id int,
  c_b_id int
)
GO
CREATE INDEX IX_table_c_a_id ON table_c (c_a_id)
GO
CREATE INDEX IX_table_c_b_id ON table_c (c_b_id)
GO

The tables are not modified after filling initially. I'm the only one querying them. They contains millions of records (table_a: 5M, table_b: 4M, table_c: 12M), but using only 1% gives similar results.

Edit: I tried adding FOREIGN KEYs for c_a_id and