Every article says the ceremony is being moved inside due to the temperature, but you are positive it has something to do with security.
This is Trump's statement:
"The weather forecast for Washington, D.C., with the windchill factor, could take temperatures into severe record lows. There is an Arctic blast sweeping the country. I don't want to see people hurt, or injured in any way."
"It is dangerous conditions for the tens of thousands of Law Enforcement, First Responders, Police K9s and even horses, and hundreds of thousands of supporters that will be outside for many hours on the 20th (In any event, if you decide to come, dress warmly!).
"Therefore, I have ordered the Inauguration Address, in addition to prayers and other speeches, to be delivered in the United States Capitol Rotunda, as was used by Ronald Reagan in 1985, also because of very cold weather."
What's logical about your conclusion?