I'm not a lawyer, but I doubt that some one would sue them over allowing "Official NOAA Charts" (which happen to be free) to be used - which are what most of the S-57 format charts are.
My guess is a more likely scenario is that some marketing
guy realized that their chart revenue base was being eroded by NOAA's free S-57s and they decided to muddy the waters.
They also have all kinds of warnings about update your charts at least yearly to reduce the risk of problems.
For us in Mexico
- it's total BS as the charts are so far off. Some of the land masses don't even match the satellite
They could fix it if they wanted by using Google Earth
and/or the Mexican Topo maps which are spot on!
When you ask them - they give the inverse BS answer "We have to use official sources to avoid liability"
I've used Nobeltec for over a decade and am considering abandoning... If only someone had decent charts for Mexico!