To be fair, walking your dogs without a leash in the city of Barcelona is already illegal. You can walk past the police and they won't do anything though, unless your dog is not behaving, then they'll use the law to give you a fine for not having a leash.

So while it's setup like you say it is, in theory, what you think cannot happen is what happens in practice. Same with public drinking and other laws. The police usually look the other way unless you are abusing it somehow, being too drunk, loud or otherwise annoying. Sitting in the park with a bottle of wine is fine and won't end up with you getting a ticket, even though it's illegal. At worst they'll tell you to stop whatever you're doing, and if you follow their command, that's the end of the situation.

Maybe the big difference here between law officers in Spain (Barcelona at least) and wherever you're based, is how strict they actually enforce the laws. Here is maybe a bit laxer, which also fits in with the whole culture here, and it's represented in how law officers behave as well.

So basically the parties that are actually the problem are too hard to find, so we target everyone who fits into a similar category.

Yes, because we don’t have infinite enforcement resources we need to decide if we are going to prioritize type I errors or type II. Given the balance of harms minimizing type II errors (i.e. requiring leashes where they aren’t necessary) seems the better choice.