About Licenses, this bridge is GPL, Joomla is GPL and SMF is, well SMF.
Simplifing the GPL:
GPL says that if you add some code to a GPL program or link a program/lib to it, this code/program/lib becomes GPL too.
One important note is that GPL is about distribution and not necessarily about the use.
This means that you can do whatever you want with GPL program, alter it, patch it, mix it with incompatible licensed code (even proprietary), as long as you are not distribuiting it.
This piece from wikipedeia is relevant:
Copyleft only applies when a person seeks to redistribute the program. One is allowed to make private modified versions, without any obligation to divulge the modifications as long as the modified software is not distributed to anyone else. Note that the copyleft only applies to the software and not to its output (unless that output is itself a derivative work of the program); for example, a public web portal running a modified derivative of a GPLed content management system is not required to distribute its changes to the underlying software.
Now the SMF License stats that:
- You cant redistribute it.
- You cant sell it.
- You cant remove the copyright notice from the output.
This makes the SMF incompatible with the GPL, as you cant do any of this.
Note that in GPL you can change the output of a program and remove the copyright notice, but you cant remove it from the source code.
So, when a bridge connects (links) Joomla to SMF, it is violating the GPL and the SMF License _IF_ redistribuited.
As you, the end user, is not redistributing the SMF or the Joomla + Bridge + SMF, you are not violating any license.
A side note, if you are allowed to redistribute SMF, then you woyld be violating the GPL if you do so. However, you can use the bridge to syncronize users between Joomla and SMF and when using the SMF you link to the standalone SMF index.php instead of using the com_jsmf component. In this case, there is no linking and GPL is not being violated.
Hope I didnt make more confusion about it

-Raul Dias