SQL Developer is free, yes, not a trial version. Where did you read that?
Here's the Oracle download page and on the sampe page is the OTN license agreement for SQL Developer:
From the Agreement : "electing the "Accept License Agreement" button is a confirmation of your agreement that you comply, now and during the trial term, with each of the following statements:...
But thank you you have answered my question
Maybe not even a lawyer could love that "Oracle Technology Network License Agreement", but one key point from it regarding the use of the free SQL Developer tool is:
Our technical support organization does not provide technical support, phone support, or updates specifically for the programs licensed under this agreement. However, if you have a supported license of an Oracle database program, then the technical support organization will provide technical support, phone support for the program licensed hereunder in conjunction with the Oracle database program license.
So, ultimately, the idea is that any Oracle support you receive will be because of paid support under a paid Oracle database license.