Some ladies will start this work with an inner strength and from a position of power (the freedom to say no, the money to walk away) but many, many ladies don't. Age is a factor but social and economic status also come into play, if you are 18, no job, no money in the bank and sent to a 40 year old mans house to have sex with him in order to pay your rent it's not going to be easy to say to him "No thanks, I only fuck" when he offers you drugs. You know it's likely the end of your date and you may really need that money, there is also the possibility he'll complain to the agency or write a bad review which may cost you future money as well, and of course there is the odd guy that threatens violence or is so paranoid they will try to stop you from leaving. These are not simple decisions and low risk activities, for a lady that is in a tough situation or is unsure of her future. It's not easy to find your legs, to know how to say no to a person in a position of power and to do so in the least offensive way possible, to expect a lady to do that when she is fresh out of school and with little life experience is ridiculous, everyone struggles with saying no to their boss or clients and it takes practice to get it right.
I've been yelled at, insulted and threatened by clients that I was not willing to indulge when they offered me something, I've also had clients call agencies to complain when I told them I wasn't staying. I've also had the reverse happen where I've been on an outcall or an incall with another lady that was high, drunk or somehow incapacitated, and I've had agencies send me on dates where the previous lady was high, drunk or made the client uncomfortable because of a related issue. I've dealt with agencies that supported me in my choices and ones that didn't. I've also worked in offices where a good number of the employees were battling drug or alcohol addiction so it's not something limited to this industry.
I think it's important to keep in mind who you are dealing with and why they are there. If you are in a position of power over the person then be aware of it and respectful of your position just as you would like those you deal with each day to do for you.