Yes, it may be legal. Yes, it may grow your company. Yes, you may not have to pay any consequences until you're big enough to pay it off as a "tax".
But, no, it's not ethical, and yes, your investors will worry that you'll shamelessly cheat them at the first opportunity.
I also don't think it's inherently dishonest to do things like offer referral bonuses for getting the competition's drivers to join your service.
Were I an investor, where would I get the impression that they would shamelessly cheat me at the first opportunity? All they've done is improve the likelihood that the business I have invested in wins out.
But, often a company has very forgiving cancelation policies to be customer-friendly. By abusing those cancelation policies to do a DDOS-style attack is going to lead to adverse impacts on the ecosystem (less forgiving cancelation policies, drivers & riders wasting time chasing bogus reservations, etc)