I'm not sure that the problem is admins - I go through the A7 noms and delete maybe 2/3s of them, and a good chunk of those I do delete aren't A7s, but are copyvios, spam, what have you. It would be nice if new article patrollers understood the point, though.
And that is why the delete button is only available to admins. If more people were capable of correctly determining whether or not an article should be speedied, more people could be given the delete button. While it would be nice if new article patrollers would tag things correctly, it's not a serious problem. The speedy deletion procedure is designed to compensate for people that don't know what is and isn't speediable by requiring an admin to make the final decision.