ADA compliance is almost never cheap or easy. That was the knock on the law when it was being debated and any business owner will tell you it's still every bit the costly nightmare people said it would be at the time. It's a terrible law and most of it should be repealed (the rest should devolve to the states, which unlike the federal government have the constitutional authority to impose it), but the fact remains that courts have upheld it and it is the law. There's no reason one taxi company should have to follow it and not another.