PSA: Please stop demonizing what she's doing as censorship. She's a private entity. She has the right to express her disagreement with GTA. It might be stupid, but she has that right.
Censorship is generally only regarded as bad when done by a governmental entity.
People boycott stuff all the time, for good reasons and for stupid reasons. A private entity's act of calling for a boycott isn't in and of itself reprehensible. People in Montgomery boycotted the buses when black people were forced to sit in the back. People didn't say "OMG THEY'RE CENSORING THE BUS SYSTEM'S RIGHT TO BE RACIST."
All she did was exercise her right to free speech (which encompasses the right to stupid speech) by saying, "me and these thousands of other people will stop shopping at Target if Target sells GTA V." She has that right.
Ironically, to suggest that people CAN'T call for boycotts of retail establishments for whatever reason, stupid or otherwise, is effectively saying that THEIR speech should be censored.
TL;DR - Boycotting over GTA V is stupid. But the right to call for a boycott is derived from the right to free speech, which is a good thing.