The thing is, there is a tension between "customers don't want to be spammed" and "spamming customers produces appreciable and measurable results". For every N disengaged customers you annoy with the unsolicited mail, you re-engage M who simply forgot about you. Tuning that ratio is hard, and sadly poorly suited to advice like "don't send spammy reminders".
I'd conjecture that "lots of spam" probably increases short-term engagement, but if these people have to be emailed very frequently to use your product, are they really interested in using your product over the long term? Are those the kinds of customers you want to have?
Yes. At least in this example, (if it succeeded) you'd end up with a website on their service after this. Regardless how hard it was to get you there, you no longer are a customer that has to be emailed frequently to use the product. You are now just a customer.
