It's not just signed posters, they continue to do handshake events at the concerts:
12/19 & 12/20 handshake events
http://www.helloproject.com/event/detai ... 0250c5390/http://www.helloproject.com/event/detai ... 370fb564e/But those are, for now, the last scheduled handshake events, there are no more listed on the Hello! Project website. That could mean that this extraordinary chart run could come to an end soon.
All these events might not always be directly profitable, but this is clearly done to create a fan base for the group. Get enough people to meet them often enough and they will stick with them. It doesn't really do anything for foreign fans, but the local fans will not be disappointed that they can meet them so often.