One may abuse it if they allow voting of older post.
Let's dig it little more to know the reason.
steemnow takes the average price of steem over last 3.5 days. That is why the actual price of steem and the price showing in steemnow are different by some amount and this amount sometime varies with a big gap if the price drops/rises sharply.
For example, now the steem price is 0.42 usd but steemnow is showing as 0.5 usd. So there is a difference of 0.08 usd and all the voting in steemit will be based on the value of steem as shown in steemnow(average price) and not the actual steem price.
So let's say it is rising market and steem is making higher highs and the price is sky rocketing, then steemnow price may be lagging behind by significant amount than the actual price and if the bots will allow voting for older posts then user will have the opportunity to predict the actual payout very closely and accordingly he will make use of bots at 4th day or 5th day or 6th day or just before payout and he may use the bot extensively to make the ROI better. Steemit is not a ROI based social media platform and the bots are exclusively for promotion purposes so as to get visibility and for trending.
As of now most of the bots restrict a user to make use of bot within first 3.5 days and this 3.5 days is kept as the barrier so that the user can not use a bot for ROI and in first 3.5 days it will be quite harder for a user to predict accurately about the payout an hence it can not take a risk to use the bot heavily to generate ROI.
I think this rule is implemented because of a couple of different factors.
For one, it's a protection against gaming, and sharepool rape.
Second, I think this rule was imposed by @grumpycat
Some agree with the rule, others don't. If the idea of protecting against possible gaming, and sharepool rape are ideas that are attractive to you, then this rule shouldn't be an issue, but some people feel there should be no rules, unspoken or otherwise.
With this topic, you will find many conflicting opinions. I personally, have no problem with the restriction.
The main reason for not promoting older post is that people who vote first always earn more steempower for healing then by voting they would earn less in healing steempower but if you realize that most have that defined as they are values By default they are programmed into the clear code that these values can be changed. Another reason is to avoid spam and people constantly send offers to these. giving the possibility to other people to be able to enter their offers and make the most of the platform.