No, people are not disrespecting non-Christian religions when they use BC, because AD and BC are simply ways of measuring time. These ways of measuring time have been used for thousands of years. They are historical ways to measure time, that everyone alive today has been using for all of their lives It is historical, not a statement of religion.
I feel that people are not disrespecting Christians when they use the term BC because BC simply means "before Christ". Even Christians know that there was a time when Christ had not come to this earth. This means that using BC should actually be showing respect to Christians. Unless the are using it to represent the current time. Then it is still not disrespectful, it just means they have a different view of what happened.