If Jen or Zack get word of Justin's plan to vote to evict Eric, it would be best for one of them to switch their vote which would save Eric and turn everyone on Justin because they'd clearly assume he just played everyone and was in fact the one who saved Eric.
They have to assume that they are going to be the prime targets after Eric is gone anyway and this way they can buy themselves at least two weeks as everyone goes after Justin and then after Eric again the following week.
I know there are a lot of Eric haters out there and for good reason, but don't we all want the possibility of America being the deciding vote for the final winner. Imagine the final 2 being Dick and Jess and the votes being tied 3 to 3 with Eric's vote left... Jess would think she had it won but then the twist that he has America's vote and it turns out we want Dick to win it all. Hopefully he holds on for at least one more week and gets on the jury.