The equivalent scene in the book is as follows, and sheds a bit more light on Lupins thinking.
want to know how it fell into your possession. I am, however,
astounded that you didn't hand it in. Particularly after what happened
the last time a student left information about the castle lying
around. And I can't let you have it back, Harry."
Harry had expected that, and was too keen for explanations to protest.
"Why did Snape think I'd got it from the manufacturers?"
"Because...," Lupin hesitated, "because these mapmakers would have
wanted to lure you out of school. They'd think it extremely
Lupins primary concern is not Sirius getting his hands on the map (Sirius has no real need of it, he knows all of Hogwarts and can stalk Harry in dog form, and Lupin should know this), but that the map will draw Harry out of Hogwarts, where Harry would be unprotected. Worse, Sirius knows all these routes and is likely using them.
While Lupin isn't (or shouldn't be) concerned about Sirius getting the map, he's also not happy that Harry didn't consider it a possibility - as @Jenayah points out in the comment, Sirius had previously gained access to the Gryffindor dorm because Neville left a list of the passwords out. Harry really should have learned from this, but kept freely using the map regardless, despite knowing that it listed several hidden passages in/out of the castle.
Interestingly, the film seems to be combining two different sets of dialogue here (Lupin and Snape, of all unlikely pairs!). The scene continues as follows:
"Do you know them?" said Harry, impressed.
"We've met," he said shortly. He was looking at Harry more seriously
than ever before.
"Don't expect me to cover up for you again, Harry. I cannot make you
take Sirius Black seriously. But I would have thought that what you
have heard when the dementors draw near you would have had more of an
effect on you. Your parents gave their lives to keep you alive, Harry.
A poor way to repay them, gambling their sacrifice for a bag of magic
And earlier in the book Snape tells Harry
"Your father didn't set much store by rules either," Snape went on,
pressing his advantage, his thin face full of malice.
Totally reasonable and in-character, Snape's dialogue coming from Lupin. Absolutely fine.