Yes this is thoughtless.
When I know someone is watching pennies I always offer to pay for ourselves, and I would definitely prefer (and not mind at all) if they ask to do this and make that clear from the outset.
On the other hand, I have a friend (not actually so badly off at all tbh) who watches what everyone eats and if someone gets even one thing more, she says loudly, in a tone that is both aggrieved and a bit aggressive: "well if that's being added to the bill, I might as well get another drink/ dessert/ coffee as well" every time someone orders something like a coffee.
I guess it irritates me because a) she ought to know by now I don't just split down the middle regardless and b) it makes it clear she's less worried about total spend in and of itself, and mostly worried about the risk of (horror above horrors!!) subsidising someone else, even if it means her paying more to avoid that risk.
It comes across as mean-spirited and to be honest a bit controlling - as though what she really wants is no-one else to have what they want to suit her. I don't know why she doesn't say at the outset something like "Well you take whatever you fancy and don't let me put you off if I skip the starter; I'm watching my expenditure atm," or something.
Because I would never have split it equally when I know someone present is watching their spending and I have never caused her to subsidise others, I always have a slight sadistic pleasure in saying at the end "well, let's all pay for what we ordered shall we!"
So I think its perfectly ok to not want to subsidise others, but there are ways and ways of handling it nicely.