For the past 5 months our site has been receiving hits from these 4 sites below;

sheratonbd.com
newsheraton.com
newsheration.com
newsheratonltd.com

Typically the exact url they come from looks something like this;

http://www.newsheraton.com/ClickEarnArea.aspx?loginsession_expiredlogin=85

The spam bot goes to our homepage and stays there for about 1 min and then exist. Luckily we have some pretty beefy servers so it hasn't even come close to overloading our servers yet. Last month I started blocking the IP address's of the spam bots but they seem to keep getting new ones everyday. So far I have blocked over 200 IP address's, below are a few of the ones I have blocked. They all come from Bangladesh.

58.97.238.214
58.97.149.132
180.234.109.108
180.149.31.221
117.18.231.5
117.18.231.12

Since this has been going on for the past 5 months our real site traffic has started to drop, and everyday our orders get lower and lower. Also since these spam bots simply go to our homepage and then leave our bounce rate in analytics has sky rocketed.

My questions are;

Is it possible that these spam bots are affecting our SEO? 60% of our orders come from natural search, and since this whole thing has started orders have slowly been dropping.

What would be the reason someone would want to waste resources in doing this to our site? IP's aren't free and either are domain names, what would be the goal in doing this to us? We have google adwords but don't advertise on extended networks nor advertise in Bangladesh since we don't ship there so they are not making money on adsense.

Has anyone experienced anything similar to this? What did you do and what was the final out come?