Cactus Game Design Message Boards

For the same reason A-Pole isn't allowed -- it is an evil object that only part of the temple for a short time.

REG > Temple Artifacts
Several Redemption® cards refer to artifacts that are “temple artifacts”. The reference to temple is general and includes Solomon’s Temple, Zerubbabel’s Temple, and Herod’s Temple. It does not include the Tabernacle. For Tabernacle artifacts, see Tabernacle Artifacts. The following cards refer to temple artifacts:
•      Filling Zerubbabel’s Temple (FF), Jehoiada the High Priest (Pi), Solomon's Temple (Pa), and Zerubbabel’s Temple (FF)
The following artifacts can be regularly found in the temple:
•      Altar of Burnt Offering (Pi), Altar of Incense (E), Altar of Incense (Pi), Ark of the Covenant (Ki), Ark of the Covenant (Wa), Book of the Covenant (P), Book of the Law (Pi), Holy of Holies (DL), Holy of Holies (DU), Holy of Holies (Pi), Lampstand of the Sanctuary (Pi), Pot of Manna (P), Table of Showbread (Pi), Tables of the Law (G), Tables of the Law (Pi), Tables of the Law (Wa), Temple Veil (F), The Bronze Laver (Pi), The Silver Trumpets (Pi), and Windows of Narrow Light (P)
NOTE: Several items may be periodically found in the temple, but are not considered part of this category (e.g., Asherah Pole (Ki), Priestly Breastplate (CL), Priestly Breastplate (CU) or Priestly Breastplate (Pi), Priestly Crown (E), Urim and Thummim (Pi)

Would altar of ahaz count as an idol?

No.  The altar itself does not meet the criteria, so the card cannot count as an idol; the art does not contain an idol, so it does not "depict an idol."

A card is considered to be an idol if it represents a statue or physical representation of a false god.

In this case, an idol is a statue or physical representation of a false god or a being found in a false religion.

Now, perhaps it is a card with false religious practices, though we'd have to discuss that.  The section itself is just them building an altar in the shape of the Assyrian king's, but the later sections have them sacrificing on it (false religious practices).  Maybe it belongs in that section.

So worshiping demons wouldn't work with alter of ahaz?

Worshipping Demons also specifies "evil altar," which lists Altar of Ahaz in the REG, so it works.

Also, the card itself says "false religious practice"...so that should probably be added to the REG list, unless I missed it there.