Our free tier and promotional hours are given at our discretion, and as such we do not allow them to stack with other subscriptions. To draw an analogy for you, AWS offers only t2.micro instances in their free tier. If you want any more resources than what is offered by the t2.micro instance, even if it's 512MB more RAM or one more vCPU you have to pay the full price of a higher tier instance as opposed to just paying the difference.
You don't have to pay anything to provide bots for free. You have up to 400 hours a month to do so. If you find yourself testing your bots for more than 400 hours a month then you are either writing brittle code or you are using it for more than just testing. If the latter is the case then you should compensate the platform that we provide for free for reasonable usage. Neither playing the victim nor the saint will get you far.