You know how some people say they are vegetarian but it turns out that while they don't eat meat they do eat seafood. And actual vegetarians have a problem with this as some people then think that vegetarians eat seafood and it all gets confusing for people trying to understand what a vegetarian actually is.
For practical purposes there may be times when a pescatarian finds it easier to say they are vegetarian presentationally because they strongly feel they align to the not eating meat bit.
But saying you are something does not mean you are.
And vegetarians should not be expected to eat seafood because some pescatarian say or think they are vegetarian.