The whole thing with "support companies you like, provide positive feedback to business models and ideas you support." If you, as a consumer, support a method of advertising and want more companies to do it in the future, there are two main ways of encouraging it: writing a letter, or buying the product. Writing a letter and not buying the product encourages them, but doesn't factor into cost analyses; buying the product factors into cost analyses, and both writing a letter and buying the product (and mentioning that you bought it because of the advertisement in the letter) provides ammunition for anyone at the company who supports the advertising method.

It's like the people who spent upwards of $500 on the Humble Indie Bundle: providing positive feedback to encourage things like it to happen in the future.

Treating it as a donation doesn't make sense, of course, and I have to agree that advertisements shouldn't be considered art, except in the most dramatic examples. Perhaps the meaning of art is a bit narrow, though; advertisements can certainly be artistic.