Is it normal that journals strongly suggest a professional English editing service? It is quite common for me to receive emails from the editors of a journal asking me, before or after the referees have given their approval, to review and improve my English. Then, the editor suggests a professional English editing service for this purpose.
My question is: Is this common practice, independently on the quality of the English of the manuscript (as an attempt to sell a service), or is it only for the cases where the English really needs improvement?
Here is an example of such an email where the reviewers had no further comments and recommend acceptance, but the editor asks for an English review:

The reviewers judge the technical content of your revised manuscript satisfactory. The English, however, is awkward, and needs improvement. Reviewer comments are included below and/or are attached.
The language quality must be improved. We advise that you seek assistance from a colleague or have a professional editing service correct the language in your manuscript, which can then be resubmitted to us.
AIP and the JAP recommend Edanz for authors who wish to have the language in their manuscript edited by a native-English speaking language editor who is also a scientific expert. Edanz is a global editing service with offices in Japan and China. Use of an editing service is neither a requirement nor a guarantee of acceptance for publication. Please contact Edanz (http://www.edanzediting.com/aip) directly to make arrangements for editing and to receive a quotation regarding price and time.
Please edit the ENTIRE paper.
Please indicate how the manuscript has been revised. Either include a list of changes that addresses each point indicating how the manuscript has been revised as a separate document titled, Response Letter or submit a copy of the manuscript with the exact locations of the revisions titled, Marked Manuscript. That will enable the editors to see whether you have complied with the reviewer comments.
In one journal where I am familiar with the editorial workflow, the review form explicitly asks reviewers to rate the language quality of the manuscript. Based on this rating, the editor can tick an item similar to "Needs language revisions" when putting together the decision letter. The decision letter will then contain a paragraph that advises the authors to do a language revision. I am not sure at the moment whether a particular service is being recommended there.

The point is that such a recommendation can get into the letter easily, but will not be included by default. Probably the editor only wrote the first two or three sentences of the letter, maybe without thinking carefully about the exact formulation, and the rest is based on a customizable template. Nevertheless, it usually means that at least one reviewer was criticizing language usage, maybe even without giving specific comments on it. I would advise you to at least double-check on language usage, and if possible have it proof-read by someone else with very good English skills or a native speaker.

However, as long as the reviewers can understand the technical content well, these points are usually not decisive for the acceptance of the manuscript. Especially, as long as any language problems are corrected, I can't imagine that the editor will care whether you use the suggested language service or not.