It was a forced referendum with un-official Russian military standing guard at the polling stations, not allowing the Tatars to vote, and limiting Ukrainian citizens from voting, caught stuffing ballots. The referendum was only recognized by Russia and it's couple of cronies.
Crimea's referendum was
a sham display of democracy
I don't know where you got there was no invasion from
Russian 'invasion' of Crimea fuels fear of Ukraine conflict
Russian forces dressed in Ukrainian uniforms seized all key assets in Crimea
Crimea was legally handed over to Ukraine by Nikita Khrushchev
Decree
How about the Eastern regions that are also being held by Russia? The aim is to keep Ukraine unbalanced unable to go forward and join the European Union and a financial wreck so they would come back to Russia begging for help. See Georgia current state of affairs. IMO Ukraine would give up the Crimea in exchange for the Eastern regions. But you have to bitterly fight for Crimea to get Donetsk and Luhansk and all the rebel held areas back.