You guys say right on your home page that you're "infinitely scalable." [...] The (strong) implication being that any WordPress configuration will perform better on WPEngine than on an alternative hosting platform."
I agree with your general point that, from the standpoint of the entrepreneur, "the customer is always right" should be words to live by. On the other hand, in the paas industry, it should be axiomatic, i.e., understood by all competent parties, that horizontal scaling (the "infinitely scalable" bit) manifestly will NOT address problematic architectures, n+1 queries, full table scans, poorly/non-indexed schemas, etc.
This, to me, is not blaming the customer, it's laying out in reasonable terms assumptions any competent developer or software entrepreneur should understand long before they employ a paas.
Your larger point, though, is valid: even after bracketing the given assumptions of what the responsibility of a paas truly is, better communication up front could temper expectations as to what "infinitely scalable" (a bold phrase, to be sure) really means to the individual customer. It doesn't mean a paas can fix a application that locks the datbase for a full second per query, etc. It does likely mean (I'm assuming here, I have no experience with WPEngine) spinning up new instances and handling increased loads should be Heroku-simple.
In this case, I think WPEngine is taking the hit for what they should (bad communication on the support side) and explaining in real terms what paas's can and can't do. There's a difference between "blaming the customer" and simply being honest about what the bottom-line issue is. NOT addressing this side of it may have long-term implications if another customer comes along with similar expectations. Being clear about what the client's responsibility is doesn't necessarily equate to "blame."
The problem with this is that the original post doesn't say "I have a site that was crashing and locking up all the time, so I took it to WPEngine." It says "I have a site that was running fine on a VPS at Linode, but I got tired of the hassle of managing the VPS, so I took it to WPEngine."
The original post is written by the complainant, of course, so it's possible it's subjective or slanted. But if it's correct on this point, I don't think it's unreasonable for a competent party -- in this case, a person with a WordPress site that runs acceptably on a commodity VPS -- to expect that it would run acceptably on WPEngine as well. Presumably performance killers like the ones you cite would be just as bad on the VPS -- if not worse, since it's unlikely that a VPS' operator will be able to tune MySQL, PHP, Apache, etc. to the degree that an expert at a place like WPEngine would.