The Israelites can be heard murmuring and grumbling about their situation pretty much from the beginning of their experience of freedom. They complain about the lack of water to drink, the standard of their food, and appear to harp back to the "good old days" of slavery in Egypt. Is this a profound case of Stockholm Syndrome, or was something else praying on the minds of the people at the time?
Many commentators ask the question as to what is meant by the term "free" in the sentence quoted above. Some suggest that it refers to the fact that Pharaoh, or the fishermen, gave the Israelites food literally for free, and that is what they missed.
Others, including Rashi, suggest a more philosophical question of freedom. What does "free of charge" mean, he asks? Free from [the burden of] precepts, according to the Midrash (Sifrei Beha'alothecha 1:42:5).
In Egypt the nation had not yet been given the Torah or the Ten Commandments, and so therefore had no moral or ethical code, no rules and regulations, by which to live. At this time of year, just after the festival of Shavuot, Jews everywhere have been celebrating the Giving of the Torah. However, some might claim that it was once the law was given to the people of the desert generation, that the grumbling really started.
Perhaps it was the freedom from rules and regulations that the people missed in Egypt, the freedom to live her lives without the yoke of God. The manna that God provided came with a very high price, the price of observance of God's law.