It is a common practice for companies to obtain patents that infringe on other patents
that are owned by same company. The Patent Office allows this. Check the cited references,
all but one are owned by NI. This practice forms a web of patents that makes it hard to
design around a particular patent.
Part of the reason that their GPIB devices are so expensive is that they contain patented
technology (now expired) as well as trade secrets. It is the trade secrets that make their
products better. Trade secrets don't expire.
Specifically, think of all the GPIB instruments that have imperfect implementations of the
GPIB standard. NI and HP have done the tedious work making these weird instruments work
with their products. That is what all the open source and other third party developers struggle
with. That large body of knowledge is the trade secret contained in these devices and that is
why they can charge the prices that they do.