Its the Optimizer to choose the best cost effective plan. However, there are maultiple conditions underwhich SI will not be used. You may need to read more of USI/NUSI selection criteria.
To use NUSI, the indexed column should be used in WHERE condistion.
Since you have placed no restrcition on any of the tables, it will obviously go for full table scan.
It is not ever possible to test NUSI usage on small data. If the data is only a block or two on each AMP, it is always cheaper to read the data blocks directly than to read the index structure, then go read the data blocks.
And as stated above, when the index is not qualified it is unlikely it will be used in a query. In general, the qualification has to be selective enough from the index so that fewer data blocks will be read than if the scan was performed. Otherwise, it is cheaper to scan than to read the index and then read all the data blocks anyway.