See Chapter 12 of our "Becoming an Expert in Conjoint Analysis Book", the 2-Log Likelihood test: http://www.sawtoothsoftware.com/169-support/technical-papers/cbc-related-papers/2033-statistical-testing
Sections 12.2.b and 12.2.e of that chapter.
Indeed, every time you add new terms to the model (whether useful or not), the fit statistic (log-likelihood) improves. This statistical test checks whether the additional terms you added to the model (such as interaction terms) provided statistically significant improvement to the fit.
Most of our users employ HB for their final model. Interaction terms that are found to be significant from aggregate logit analysis usually go away and are not significant under the HB model (which estimates utilities for each respondent rather than an average across groups of respondents).