no - that's by design. There could be any number of reasons you don't want that: You're importing multiple files and don't want to get blocked by google scholar, you don't have an internet connection open at the time, you know it's not going to work (bc the pdf is not ocr'd for example) etc.

I don't know exactly how (it's closed source after all), but partly by relying on the collected data from all other people who have uploaded data in the past - personally I feel quite uneasy about the way in which Mendeley takes ownership of your data to provide a service and I'm not even sure that Zotero would be allowed to do that under its terms of service.