Almost without exception, hotels are open to non-guests. It's in their interests to encourage use of facilities such as restaurants, bars and shops. Disney is no different in that respect and actively promotes such activity. It follows that some abuse of non-chargeable facilities, particularly pools, will occur (and I'm not just talking Disney; there are some fabulous pools at the upmarket hotels around Orlando). Luckily, most people don't misuse the pools or parking and, on the whole, the scale of the problem is minimal and has no discernible impact on paying guests. At those resorts where it is more widespread, Disney has put in the necessary controls to prevent it. Universal has apparently had to introduce similar measures recently, albeit I think the issue was with locals rather than tourists.
When Animal Kingdom Lodge first opened, there was a policy of not allowing access just for "looking". You had to have some kind of reservation (either room or restaurant). I can understand that - the place would have been mobbed. It didn't take long for the novelty to wear off, though, and the policy was revised. Like others here, we've visited most of the resorts (whilst staying both onsite and off) with the stated purpose of "having a look around" and we've always been welcomed.
ETA: Almost all hotels in the area offer free valet parking for those visiting to eat - that's how much they value the custom of "outsiders". Disney offers the same incentive for those with the Tables in Wonderland card. Without this income stream, the rooms would be a lot more expensive.
