Watching Crimeans dancing in Lenin Square after voting overwhelmingly to breakaway from Ukraine must have irked US and EU leaders responsible for responding. The people of Crimea saw an opportunity to finally sever ties between a corrupt insolvent inept and unstable government and they took it. Russian influenced or not, most of Crimea felt Russia was the best option moving forward.
Coming up with a winning narrative to contradict a peaceful democratic referendum conducted by the people of Crimea is going to be difficult. As the "Western Click of Nations" prepare the first tranche of sanctions, they must take care not to overcharge "Putin the Perpetrator" since the international laws Russian officials have allegedly violated are being vigorously disputed.
Simply stating a nation state is in violation of international law does make it so. Whenever multiple nations are disputing a point of international law, devastating consequences usually follow as crippling economic sanctions and international isolation are generally first choices. When those fail, as they generally do, the military option is never far away.
Before one a can legally impose "consequences" one should have made a convincing case before an impartial arbiter. To be calling for international sanctions against a nation state and talking of providing military assistance to anybody before an issue of this international magnitude has been adjudicated may be premature as well as illegal.
So far Russia has a maintained plausible defense for its response to events in Ukraine. If Putin stands pat on on the issue of Ukraine's illegitimacy, while quietly monitoring other pockets of potential unrest (without spooking the neighbors of course), they might be able to keep the West of balance.
Simply stated, prudent measures have been taken to preempt bad actors from threatening Russian interest. Since Putin feels only Russia will determine where Russian interest begins and ends, they'll be moving "certain assets" closer to suspected hot spots. After witnessing how easily Ukraine authorities were rolled over by street thugs, it made perfect sense to be prepared for the worst.
Until a new government can be established in Ukraine by free and fair elections, Russia is reserving the right to keep a contingent of force at the ready to confront any threat to its national interest. If that makes the international community nervous, it may be something that'll have to be tolerated for the time being.
From Vladimir Putin's perspective, Ukraine as it wobbles now, is a bastard state. Kiev, represents little more than a mass of people, without leadership, money or direction. Until elections can be held and a new constitution drafted, any previous contracts, agreements or obligations, including the Budapest Memorandum are considered rendered null and void.
Bottom line: Ukraine itself became null and void when President Viktor Yanukovych was run out of Kiev by mob violence. If any other signatories to said agreements wish to continue honoring said agreements, they're free to do so of their own volition, but Russia has moved on.