Tchaikovsky didn’t ‘lift’ that tune...it was a conscious quote that all his Russian listeners would have understood. Using folk material was very common at the time. Russians were especially obsessed with it.
OK...I know what you mean, and agree. He quoted church music too. Liszt did the same thing in his “Hungarian Rhapsodies”. John Williams (and Andrew Lloyd Webber) should have attributed their melodies as well to their predecessors. As should have Paul Simon in “American Tune”. It’s OK to pay “Tribute” to previous melodies, or even to gather them together for a “Rhapsody”, but not OK to pass them off as one’s own.
Just my humble opinion as a Songwriter. When I come up with a cool phrase, I WRACK MY BRAIN to make sure I didn’t hear it somewhere before. Sometimes, I toss the phrase away because it even SOUNDS like I might have lifted it, even if it is original. Maybe I am a wee bit too meticulous, but I sure as heck don’t wanna get sued! LOL!