I can't do side work in my field of employment or work for any company that is considered a competitor in any capacity. To say it's illegal or wouldn't hold up in court is not true. If you enter the draft, they are drafting your NFL rights for a year. If a player wants to work at McDonalds, they can't stop him, but he can't work for a competitor.
The player can re-enter the draft the following year, but will most likely be taken in a later round after being out of football for a year which means 1 missed year of pay followed by a smaller rookie contract. Couple that with short career associated with football and it's hardly a good idea.
Top players could probably force a trade and lower round guys might get just get cut.