Why is this a Lewis structure of ethyl acetate?
The two rules you cite are guidelines for the 'best' Lewis structure.  However, in the example provided, all electrons are accounted for and all atoms are connected making it a 'valid' Lewis structure.  

In the context of resonance forms, this is likely a minor contributor. 

However the electrophilic center in ethyl acetate is the carbon you marked as positive, this form has some descriptive  and predictive power in discussing reactions of ethyl acetate.  In general, for $\ce{R2C=O}$ molecules (esters, aldehydes, ketones, etc), The carbocation (sextet) carbon has some relevance.