You really don't understand how available now works?
She, literally, posted an available now ad with a later time in the subject of the post. Kind of the exact opposite of being available now.
There is something really funny about someone who lists herself using an ad site's "Available now" feature at 10am, and the body of the message says she's available from 6pm until midnight and she requires advance scheduling
...even in the wee hours...whats a horny wet girl to do????...if they're real lucky they might get it for free....lol
I have to admit that I have posted that I need advance scheduling while also having on available now. The biggest reason for saying I need advance notice is to repel men who do not want to go threw a screening process. My schedule is very tough to work around so if I find myself with an available day I was not expecting I will put up the available now indicator. Previously, I have put the indicator on and previous clients have noticed it allowing us to make a last second booking.
While I understand your argument you have to look at it from a different perspective.
If you are cruising P411, and take the time to get a giggle and post about it on TER over something as silly as this, maybe you should have contacted her and gotten yourself laid.
It could have worked 
I constantly see someone online bitching that some guy called them for a short notice appointment when their site or ad clearly states that they need advance notice.
I read ads and pay attention. Just like I won't call someone who is out of my price range and ask them to lower their price, and I won't call someone who says she never takes short notice appointments and ask for one.
There are plenty of other options for people who advertise as available now and mean it. I won't waste my time on someone who is just using that to get more people to look at her ad under false pretenses.
In the case of the P411 board, they have labelled it "Available Now", but it is really an "Available Today" board. It allows you to post available hours for the day you are posting on, and even suggests you should post when you are available that day. The way it was designed to be used doesn't necessarily match what the client may be expecting to see based on the name.
I have a screening process that takes time. If I find myself with spare time and I have an incall I will use "available now" features or post on Twitter. Sometimes folks who are pre-sreened catch it and we get to have some last minute fun.
But yeah, most providers just use it as another way to get their name out there.