Rationale refers to the reasoning and justification behind human decisions, opinions, and beliefs. In software engineering, rationale management focuses on capturing design and requirements decisions and on organizing and reusing project knowledge. This paper takes a different view on rationale written by users in online reviews. We studied 32,414 reviews for 52 software applications in the Amazon Store. Through a grounded theory approach and peer content analysis, we investigated how users argue and justify their decisions, e.g. about upgrading, installing, or switching software applications. We also studied the occurrence frequency of rationale concepts such as issues encountered or alternatives considered in the reviews and found that assessment criteria like performance, compatibility, and usability represent the most pervasive concept. We then used the truth set of manually labeled review sentences to explore how accurately we can mine rationale concepts from the reviews. Support Vector Classifier, Naive Bayes, and Logistic Regression, trained on the review metadata, syntax tree of the review text, and influential terms, achieved a precision around 80\% for predicting sentences with alternatives and decisions, with top recall values of 98\%. On the review level, precision was up to 13\% higher with recall values reaching 99\%. We discuss the findings and the rationale importance for supporting deliberation in user communities and synthesizing the reviews for developers.