You should only vote up/down on answers with respect to their quality, not whether the answer involves commercial software or open source. If the OP specifically asks for open source or free software, but the only viable alternative is a commercial product should I then not answer the question? If I felt that using the open source alternatives was really a bad choice relative to the commercial software then I'd probably mention both but indicate why I think the commercial choice is superior.
The bottom line, I think, is context. Yes, down vote spam. Don't merely assume it's spam because it happens to be a commercial product. With @Gnoupi, I would agree that full disclosure is appropriate.
On a related note, though, an answer with only a link is a poor answer, no matter what it links to. You should always include at least a summary of the linked information to cover the case where the link changes in order to keep the answer viable.