Maybe I'm reading this wrong, but it looks like there is no UCSC license included with the rtracklayer /src/ucsc tree:

@biocrusoe the bigger problem as I see it is rtracklayer redistributing the Kent source with attribution but without the license.

— Matt Shirley (@mdshw5) December 10, 2015

The Kent src/lib directory contains this README file, which clarifies things a bit:

With the exception of the gifcomp.c module, all 
modules here are free for all use - public, private,
or commercial. The gifcomp.c module contains code
originally distributed by CompuServe under a similar
"free for all" license and the Unisys LZW patent expired in 2003.

I'm happy to accept advice from anyone who understands this stuff. The code was released for "all use". rtracklayer does not include the gifcomp.c file. It only contains select files from "src/lib" and "src/inc".

everything under src/lib is in the public domain. You're fine. If you are unsure about the license, you can email us at genome-www@soe.ucsc.edu or the UCSC mailing list.