It is not spiritualizing Scripture - but may seem that way if you don't know why they arrived at that conclusion. Revelation one states this:

Revelation 1:19 Write the things which thou hast seen, and the things which are, and the things which shall be hereafter;

1) The things which thou hast seen - John's vision of Christ in chapter one.
2) The things which are - the seven churches, representing the church age according to the order of the letters, chapters 2-3.
3) The things which shall be hereafter - chapter 4 onwards, the events of the Tribulation and afterwards.

Chapter four starts with this:

Revelation 4:1 After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will shew thee things which must be hereafter.

This is after the church age - which would correspond with the church being raptured. When compared with 1 Thessalonians 4, you can see the language is similar.

1 Thessalonians 4:16-17 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.

Certainly there are some similarities - even if you do not think John's being called to Heaven pictures the rapture, Biblically it certainly fits the timing of the rapture.