The problem is that you cannot place armor stands if any entitiy is at this position. This means that this affects items, XPOrbs as well as AreaAffectClouds (particle clouds from lingering potions) and other entities where this restricition makes no sense.
This is different to normal block placing which works if there are certain entities in the way, for example items.
The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.
The reason why this happens is because the method net.minecraft.item.ItemArmorStand.onItemUse(ItemStack, EntityPlayer, World, BlockPos, EnumHand, EnumFacing, float, float, float) tests if any entity is at the position where the armor stand should be placed (method net.minecraft.world.World.getEntitiesWithinAABBExcludingEntity(Entity, AxisAlignedBB)). Instead it should probably use the same method that is used when the player wants to place a block: The method net.minecraft.world.World.checkNoEntityCollision(AxisAlignedBB, Entity) which tests if the entities which are in this bounding box prevent block placing.