You're kidding, right?

Try being a single parent from day one. Stayed home the first year with the baby...no help from anyone. Back to work after a year. Then juggle working with having a great boss who let me out of work to volunteer at school, attend plays, etc. Then stay at work extra hours throughout the week to make up the missed hours while attending things. Throw in what SAHMs do all day into the evenings. Being a SAHM is a breeze! If not, maybe you haven't figured out the most effective way to do it.
Sheesh, I wanted to stay out of this one, but enough is enough. You seem to have a major chip on your shoulder and just won't take JoiseyMom at her word. In her opinion, and in her experience, being a SAHM was harder. Others who have worked outside the home and also been a SAHM thought being a SAHM was more stressful. For my part, I can't say, since I haven't worked since DD came along. I'm not so sure I'd have the same opinion, but I'd at least respect that THEY found being a SAHM to be harder and more stressful. By the same token, I'd respect that you found the opposite to be true. Reasonable people can differ on this point, you know.
This whole thread is a tempest in a teapot. The two moms in the OP's post were planning a Halloween party and the majority of the moms helping with it were SAHMs who found that meeting during the day suited their needs best. The OP had nothing to do with the Halloween party.

When it's her turn to do the Christmas party (or whatever it's called these days) then she can have an evening meeting if that's convenient for her. If the other moms want to help with that party, they'll find a way to make it to the evening meeting. Problem solved.
If a school has a high percentage of working parents (and usually the moms are the ones involved in the PTO) and they want an active PTO, then they'll have evening meetings. At our school, most of the moms are SAHM and evening meetings would not be a hit. Too many traveling husbands, no one wants to get a sitter when they're at the school volunteering so often anyway, and the kids have lots of homework and activities after school. It would just be a hassle. Organizing parties, luncheons, field trips, etc. is mostly done by email. We don't need to meet for that. If a working mom/dad wants to do something like help at a party, we usually work around their schedule and give them first dibs so as to make it easier for them to join in. Last year, another mom (she had taken off work) did a project with the class and when it was over, there was a lot of clean up to be done. I hadn't realized she had taken off work, and when I did, I told her I'd finish up so she could get back to work. (I had noticed she looked a little anxious and it seems the project took longer than expected and she had a meeting coming up soon.) I figured that she'd taken off work, her child was thrilled she'd been there to help, and what would it hurt me to clean up all the stuff so she could get back to work? It only took me 15-20 extra minutes or so, but I had the flexibility and she didn't. The way I saw it, I was not only doing her a favor, I was really doing it for the kids. I wanted to make it easier for her to come in and help, not harder, because the kids love it when parents show up for class.
I don't know why it has to be working moms vs. SAHMs, when really it shouldn't be about the
parents at all.