The Class B (or A, for that matter) require an integrated GPS. They are tested, certified and approved as an integral assembly.

In fact the problem is that your AIS's NEMA information may get treated as "your" GPS coordinates. Some programs and devices allow you to chose which GPS is "read" -- some take the first one, some just take each NEMA sentence (which would cause some anomolies at recorded positions.)

Since GPS units have variations in accuracy and wooble in readouts (whether they be measured in inches or yards)