Why must vegetarians be careful about the amount protein in their diet?

Hidden meat products are in apparently vegetarian products--for example Yoplait yogurt has Gelatin in it which is derived from animal tissue. Hot Cheetos probably has pork flavoring (except the giant puffs).

Also, vegetarians need to be careful not to overdo the simple carbohydrates (easy on the ice cream!) and get enough protein, iron, and vitamin B12.