In order to communicate utility and reliability, the logo was made to resemble not just a 't' but to appear clamp or wrench-like. There's perhaps a security connotation here as well.
There's also a few hints about Tarsnap's implementation. The blockiness of the logo relates to how files are broken up into blocks, to the general idea of composing a secure system through well-understood building blocks, and finally a 4x4 grid was deliberately chosen to be the same size as the AES state matrix.
Thanks,
Ben
