This tends to be "the" big problem with UML (and similar): Very few people care about, or have a reason to, stick 100% to UML. They want to "speak diagrams", and just like written or spoken language, our syntax and semantics is context dependent and dependent on the dialect and speech patterns and understanding of those we speak to.
So the "UML" people learn is often a dialect of sorts closer or further from the formal grammar, and a lot of tools fail to take that into account and try to force you into conformity. We accept that for programming languages, yet we still use pseudo-code in all kinds of contexts, but diagrams are one step up - one of the things we "escape to" when writing formal code is too complex or we want to explain things to people who don't know or care about the details.
There's a significant strain there between those conflicting goals of diagrams as communication, and the evolution of CASE tools that wants diagrams to be formal, that is much closer to how we deal with natural languages (people arguing over formal use vs. slang) than with programming languages, and it's fascinating.