When you register for a conference they normally ask you to give them your email address so that they can communicate with you. What also happens, most of the time, is the vendors and sponsors of the conference also get your email address.
I don't totally understand giving all the vendors my email address because there may be some vendors that don't meet the needs of my company or my clients. The reason I don't like this is because the vendors also normally have the scanner where they can scan your name badge to get your contact information. If they are scanning you it means that you went to the booth and made contact with someone.
Recently I missed a conference that I had registered to go to because of other commitments. Now I am receiving emails from the various vendors thanking me for attending the conference and also stopping by their booth. It is amazing because I know I didn't go to the conference so it is frustrating to get all these emails that if the vendors had done the work correctly I wouldn't be receiving.
In the past I also received phone calls from vendors wanting to know if they could assist me and my organization by answering questions. Normally during this call they also thank me for dropping by their booth at the conference. The funny thing is some of these vendors are the booths I intentionally didn't drop by at the conference because they don't meet the needs of my company or my clients needs.
So, if vendors really want to do their job they should only be contacting the attendees that dropped by their booth and not the entire group of attendees or worse the group of registered attendees.