Creating a situation that is too easy to cheat

Solution 1:

You may call it an open invitation.

TFD:

an open invitation

2. Something that provides an ostensible reason or excuse for someone to do something immoral or illegal.
A lot of people fear that legalizing marijuana will create an open invitation for people to try, buy, or sell any drug they can think of.

Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

Solution 2:

Perhaps asking for it

(idiomatic, informal) To provoke an unwanted action.

Leaving your wallet visible on the car seat is just asking for it.

Wiktionary

Similarly asking for trouble

persist with actions or an attitude despite the probability that it will cause trouble;

He is asking for trouble with his behavior

Wordnet 3.0 (at thefreedisctionary.com)

Often these terms are used in connection with misbehavior as well as careless behavior.

Solution 3:

I get the sense that you want a term that includes the idea of transferring responsibility from the perpetrator to the victim, because the victim was so reckless.

Taking excess risk is sometimes called courting disaster, but it doesn't require a perpetrator or victim. Riding a bicycle with your eyes closed would be courting disaster.

There is also the concept of an attractive nuisance. If you have something fun like a pool or a trampoline in your backyard, and a child trespasses and hurts themselves, you can be sued, even though the kid broke the law. That sort of transfers responsibility, but not exactly the way you describe, since the victim and perpetrator aren't the same.

In the case of leaving your keys in your car, you could say practically giving it away.

Bib's asking for trouble is pretty good. For emphasis, I sometimes use the phrase "begging for it".

Solution 4:

In German we say "Gelegenheit macht Diebe", and Leo suggests a translation, "opportunity makes the thief"¹. The phrase is not restricted to actual theft. It describes the observation that most people do not actively pursue anything criminal. But yet some (many? most of us, given enough incentive?) are not able to resist an opportunity which presents itself, like the key in the ignition.

It is usually used to explain why something happened, or as advice that simply not tempting people suffices to avoid most harm. For example, putting the key in the unlocked glove compartment, out of view — but still trivially reachable! —, would keep the vast majority of people from stealing the car because they would not actively search the car.

(I see only now that NVZ made that remark long before I wrote this. Well, I think it's a good answer.)

Solution 5:

When done by private indivuals, the other answers are appropriate; but when done by law enforcemnt agencies, the word is entrapment