I'm with [MENTION=54629]pukunui[/MENTION] on this: you want the fruit of many hours of an artist's labor, you pay for it.
These maps aren't just sketches the artist whipped out in one afternoon, and it is ridiculous to treat them as such by demanding them given to you free.
While I think it's awesome that Wizards is doing these previews as part of the charity, and fully support it, I will argue this point a little...
Once the charity event is over, and the book released, there is absolutely ZERO reason for them to not offer the map for free. Bold statement? Consider this:
1. RPGs are played at a table, handouts a given. A map of an area that might be used in the game is a tool that should be available to playing groups.
2. But why FREE? Well, consider that Wizards is the company that has stated unequivocally they have the resources to do (reasonably) whatever they want. That includes paying their artists and cartographers the right amount of money to...
3. ...they've offered these sorts of maps for free before. In every edition since the internet was a thing. Before then, "you can photocopy this for personal use" has always been a thing. Fan sites and messageboards and wiki articles tend to utilize these sorts of tools.
I'm not advocating for all art always to be given away for free after the release of a book. Not at all. But the tools that are immediately useful to a vast majority of tables should be. Character Sheets (already done!), maps (nope, you have to pay extra), and -- if they were using them -- cheap tokens that are adventure-specific should all be offered for free in case whatever hardcopy you own is damaged, or you don't want to write all over it, or if you don't have access to good scanner/photocopier (especially if you're disabled), etc.
It's definitely an opinion, but it's one a hold to strongly given past experiences with Wizards and many, many other game companies. This change to giving the artists more money is on the one hand fantastic for the artist and certainly great for special versions of maps and whatnot, but having an adventure or campaign packaged with materials which you are then given an additional (monetary) barrier to really make use of is, IMHO, a bad business practice.
Pay your artists properly and this won't even be an issue, if adopted.