This relates to a change made in the third edition of The Hobbit.
In the original text of The Hobbit as first published in 1937, there was no reference to them camping before, and thus it made sense to say they never camped before.
1937 First Edition
These are how the two quotes read in the 1937 first edition.
...and sang songs Bilbo had never heard before. Inns were rare and not good, the roads were worse, and there were hills in the distance rising higher and higher. There were castles on some of the hills, and many looked as if they had not been built for any good purpose. Also the weather which had often been as good as May can be, even in tales and legends, took a nasty turn.
To think it is June the first tomorrow,...
...They decided in the end that they would have to camp where they were. So far they had not camped before on this journey, and though they knew that they soon would have to camp regularly, when they were among the Misty Mountains and far from the lands of respectable people, it seemed a bad wet evening to begin on. They moved to a clump of trees....
1966 Third Edition
In 1966, when making the third edition text in response to the Ace Books controversy, Tolkien added in a mention of them camping, and at the same time removed the later statement that they had never camped before.
...and sang songs Bilbo had never heard before. Now they had gone on far into the Lone-lands, where there were no people left, no inns, and the roads grew steadily worse. Not far ahead were dreary hills, rising higher and higher, dark with trees. On some of them were old castles with an evil look, as if they had been built by wicked people. Everything seemed gloomy, for the weather that day had taken a nasty turn. Mostly it had been as good as May can be, even in merry tales, but now it was cold and wet. In the Lone-lands they had been obliged to camp when they could, but at least it had been dry.
To think it will soon be June!...
...They decided in the end that they would have to camp where they were. They moved to a clump of trees...
In The Annotated Hobbit, Douglas Anderson points out that most of these changes were to bring the book more in line with The Lord of the Rings, because "in terms of the geography of The Lord of the Rings, they had long passed out of the lands of respectable people" and so there would already be no inns and they would have already been needing to camp. Also Tolkien adds the word "Lone-lands" as "a linguistic equivalent of the Sindarin Elvish name Eriador" used in The Lord of the Rings.
However, of note, not all of the third edition corrections were made at the same time
There were a few different editions of the third edition released, and each seems to have been given a slightly different set of corrections depending on what was available at the time.
The first one to be released was the Ballantine Books paperback edition in the US, in February 1966. This was the most rushed, as there being a new authorized edition in the US to compete with the Ace Books edition was the main purpose of the revisions being made in the first place.
A few months later, in June 1966, there were a corresponding hardcover and paperback edition released in the UK by Longmans and Unwin Books.
The Ballantine Books edition only had the first of the two changes discussed here. Its text persisted through reprints for many years, even as far forward as the edition you've said you have which seems to be this one from the late 1980s.
So in conclusion:
- In the final corrected and revised third edition text they had camped before.
- In the original first edition they had not camped before.
- In the edition you have they had camped before, but there was a reference to the non-camping from the first edition that had incorrectly not been removed.
For more information, see The Annotated Hobbit pages 66, 68, and 385, and J.R.R. Tolkien: A Descriptive Bibliography, pages 28-44 and 64-66