Its a reciprocation tool. But it does give people choices to retweet the messages they like. They don't have to retweet all the messages. They can read 20-30-40 (upto unlimited) messages and select just 4 of them to be retweeted per day. If they like none of the messages - they retweet none.
So how is that spamming?
Right now, which tweets are not liked by users is being recorded. But the data is not being used to rank the tweets. In the future maybe.
Also, if the app succeeds, in the future - may add categories or tags or something too. So folks who like web development will only see web dev RTs. So folks will get to read more relevant RTs. But right now, adding all these features would be an overkill.