Russia and Armenia are strategic military treaty allies.
Based on the provisions of the Treaty between the Republic of Armenia and the Russian Federation on the Status and Operating Conditions of the Border Troops of the Russian Federation on the territory of the Republic of Armenia of September 30, 1992, Border Control Department of the Russian Federal Security Service safeguards the state borders of Armenia with Iran and Turkey jointly with the Armenian border guards. The 13th session of the Intergovernmental Commission on Military-Technical Cooperation was held in Moscow on October 1-5, 2018 (the previous one took place in Yerevan on September 5-8, 2017)
Essentially, Armenia helps Russia project power beyond its borders and not limit it and in return Russia supports Armenia's strategic objectives.
Russia's objections to NATO expansion
The fight is between NATO (remember Turkey is a NATO member) and Russia for control over the Baltic Sea and the Black Sea.
Finland and Sweden
- Russia shares the Baltic Sea with Finland and Sweden
- If Finland and Sweden join NATO, Russia effectively loses control of the Baltic sea to NATO
Ukraine and Moldova
- If Moldova joins NATO, Transdniestria, an exclave of Russia is in danger
- If Moldova joins NATO, it is further impetus for Ukraine joining NATO
- If Ukraine joins NATO, Russia effectively loses control of the Black Sea
North Atlantic Treaty Organization
The above countries are simply proxies between some NATO members and Russia. Not all NATO members are aligned against Russia, but the way the treaty operates, it only needs some members of NATO to start gnawing at Russia's interests to get everyone else involved.
Thus, Armenia's CEPA is not considered a threat by Russia, due to their integrated security infrastructure. While, Moldova and Ukrainian economic integration with EU is seen as the first step towards a security integration towards NATO, which Russia sees as a threat.