I totally agree with the OP. Bear in mind, she is in a strength training class - the women lifting only 2kg weights and not pushing themselves at all have chosen to do strength training! What they are doing is the opposite of with strength training.
The first culprit - aside from ignorance - is the instructor, who isn't guiding these women properly. This is a strength class, so they absolutely should be being pushed more by the instructor; otherwise, they are throwing their money away while gaining very few health benefits.
Strength training is essential for women, especially as we get older. Not only does it increase your functional abilities for everyday tasks (to carry things, pick things up from high shelves, get up off the floor, pick things up from the floor, move in a variety of ways) and increase your strength (very important), but it also strengthens your bones as well as your muscle and is extremely beneficial for your all-round health, particularly cell health and metabolism. It protects against diabetes, cancer, dementia, osteoporosis, sarcopenia, arthritis, heart disease and a whole host of other diseases and conditions. It can help deal with conditions such as arthritis (I have knee arthritis and the problems stemming from it have almost completely disappeared since I started strength training).
And for that, yes, you do need to push yourselves and lift as heavy as you can. If for some reason you can't push yourself to the max (e.g. my partner has a hiatal hernia and she has to be careful with breathing and her diaphragm) then increase the number of reps, but still always be trying to increase the load you can lift and progress to a higher weight.
When you're in your 70s, 80s and even later, you will be very grateful that you pushed yourself to the max!
But, if strength training isn't your thing, then don't go to a strength training class!