Clearly you can. A recent post in Book Business Magazine describes how McGraw-Hill Education will include free teaching resources within its paid education platform, Engrade, alongside its own paid resources, on the basis that these free resources are better curated and hence easier to find and to make use of. However, users will be charged for access to the free resources within the platform, since McGraw-Hill state they have had to pay to have them tagged. McGraw-Hill is open about what they are doing: they say they pay to have the material selected for quality and then tagged, so it is only reasonable to charge end users for the selection and better navigation.
This sounds fine to me, but I recall how there was (and still is) a rival to Wikipedia called Scholarpedia, an encyclopedia project started in 2006. Articles only appear in Scholarpedia when they have been reviewed for quality by a team of accredited experts. Anyone can suggest an article, but it is not published until approved by an approved Editor or Curator. What is the result? Scholarpedia has 671 articles, while Wikipedia has over five million. OK, the articles aren’t so reliable, but clearly many people don’t care. McGraw-Hill can point out, quite rightly, that the act of selection has a cost – but the public preference is clearly for the larger, less curated, collection. Perhaps teachers will pay for convenient access to free content – it is the institution’s money rather than their own that pays for the resources.