With regard to tax law, accounting is generally different between corporations and individuals. Specifically, individuals use cash accounting while businesses frequently use accrual accounting. Furthermore, businesses can deduct far more in expenses than an individual. One can argue about that, but I think it makes sense when you realize that corporations are ultimately owned by individuals, but individuals cannot be owned by other individuals or corporations. Individuals are ultimately the "base case" of ownership, so I think the different tax treatment of revenue/profit/income makes sense.
Most opponents of corporate "personhood" aren't really opposed to juridical personhood of corporations and other non-natural persons, they are opposed to the effect given to corporate personhood in certain domains; corporate personhood has never been fully equivalent to natural personhood in the law, and there are quite different ways in which corporations could be treated as juridical persons without directly holding the particular rights that have recently caused a lot of people to get offended about "corporate personhood".
My personal opinion is that both personal and corporate tax legislation is more or less flawed and that the(effective) tax rates probably need to be adjust but that doesn't mean that I want them to have the same legislation.
I favor a tax based on percentage of gain. If a Mega-Corp makes $30Bn a year, and I only make $30K, we each pay accordingly, at the same percentage rate.
Of course there would be a great debate about exactly what that percentage should be, but it should be equitable and not too onerous.
Why not hold corporate entities to the same standard as individuals?
Isn't the USA supposed to be about equality? (Remember, freedom & equal rights for all?)