Pumping mothers have lives too. Should they hide indoors because you don’t think it’s ‘acceptable’ for some reason?
Literally no one said that 
It's the response taken by some, because literally no one has a problem with it as long as it's done in a discreet dignified manner. It's this "I can and will expose myself as much as I like providing I can link it to breastfeeding and if anyone dares suggest it's unnecessary, it's their fault for looking, pervy bastard, and they clearly want my child to starve" tripe. Again, no one cares, it's abour being respectful to the fact other members of society are around you in a public place. And particularly on a train where it's often quite confined. No they shouldn't have to "not look". Because people only look, even accidentally, if you're drawing attention to yourself, goes for anything, not just using a breast pump. If they're looking, you're causing them to look.
Does anyone care you've slipped a pump on under your clothes/cover? Nope. Does anyone think it's wrong? Nope. Would anyone have the first idea you were using one if you did it in a mindful way? Nope. Exposing a breast on public transport which is entirely unnecessary then telling people they've got a problem if they look under the reasoning that breastfeeding/pumping is what's best for you and your child is nonsense, because it's got nothing to do with how you feed your child, again no one cares the "looks" are because you've unnecessarily exposed yourself and this caused people to look at you.
I breastfed and pumped. Both at home and in public. Never got a look. Never got stared at. Never gave people a reason too. So it begs the question, if you've got a problem with people looking, and it's so easy to be discreet, why choose not to be... because it's a choice. Your choice.
This is exactly the same as people who let their kids run around restaurants being disruptive then claim it's everyone else's problem and clearly they don't like children to excuse the unnecessary behaviour. On the "don't look" logic, if people don't want to be around your kids behaviour, they should infact leave. "It's natural. It's what children do". No. It's what children are allowed to do when the adults they are with have no consideration for the public place they are in, or the other people in it.
Feed your kid. Pump away. Have some consideration for others in a confined public space. It's that simple.