Because Zelensky is the president by popular vote of the...

Because Zelensky is the president by popular vote of the Ukranian people.
Supporters can campaign for whoever they like. That's normal politics.
Its none of Putin's business. As you pointed out, Ukraine was no threat to Russia.
The US doesn't interfere in the politics of Mexico or Canada. They are rightly upset though by Moscow's cyber hacking and interference in the US elections.