Because sometimes, your actions are either legal without permission or unplanned, and in both cases, being denied permission could be a problem. I'll use an example for each one.
Legal without permission: I've bought a device, and I'm going to run security tests on it. This device is mine, and I have that right. I do not require the manufacturer's permission to try gaining extra control of the software running on it. If I find a vulnerability in this one, I'll inform the manufacturer in the hope that they will fix it for all users of the device. If I asked them for permission to test something that I own and they declined, it would have no effect on my rights but they might think that it allows them to come after me. Manufacturers that don't want their vulnerabilities disclosed and don't want to fix them have frequently taken this approach to attempt to silence researchers who discover real problems.
Discovery is unplanned: I'm using a service legitimately and find a problem. This may be entirely accidental (I mistyped a URL, for example) basic (oh, look, this form reacts wrongly when an SQL query is put in it), or more active (look, they've got private information in the HTML of this page which they're sending to me without authorization) but in all cases, it's something that is made available for my use. Even in the SQL example, I'm putting text in a box where I'm supposed to do so, and if my message actually contains a valid SQL query, it's valid input. Having found this, I inform the company that there is a possible issue. Again, I haven't done something invasive to discover they have a problem, but if they're annoyed or don't understand what I've done, they may react badly. I shouldn't need their permission to do that.
There are many cases where you do need permission to do a test, and where failing to get it makes your activities criminal. A penetration test without permission is nearly always an obvious crime. These are pretty clear. Unfortunately, when the activity is clearly acceptable, researchers are not always treated well when they disclose it to the owner, which is why more protections are needed.