It is illegal for anyone to offer a California resident, via the Internet, a gift with the intent to offer a sales presentation, without disclosing at the time of the offer of the gift, in a clear and unequivocal manner, the intent to offer that sales presentation.
(California Business and Professions Code Section 17533.8.)
I believe the original purpose of the law was to have predisclosure of sales pitches, such as for a time-share, that would be made if someone was to get a gift.
But the law applies in other contexts, and has been amended to specifically refer to the Internet:
If you're offering a freebie report just to collect email addresses with the intent of pimping products, that should be disclosed.
If you're offering a freebie after collecting an email address, for the purpose of pitching your latest product launch, that should be disclosed.
This applies even if the product launch is not planned on being introduced until a couple emails have been sent. For example, your plan is to offer a free report in exchange for an email address. Then to send a couple more free reports over the next week about the importance list building. Then to send in two weeks an email promoting your new list building course. Since the original intent was to make that sales pitch, it needs to be disclosed at the outset.