There is absolutely no legal issue. The big three (org|net|com) are entirely advisory in nature [1]; the vast majority of customers couldn't tell you what they're supposed to advise. Google has historically used "presumed difficulty of getting this domain name" as a quality heuristic; this advantages exact-match domains on the big three (which are treated identically) and causes relative disadvantage for e.g. .biz.
Many folks on the VC-funded trajectory would tell you that there are brandability issues which would counsel you starting on, instead, getBETTERNAME.com or BETTERNAMEapp.com or something and tradition to BETTERNAME.com at any price once you have the ability to do so.
I built a company on a .org and ran it to sale, because the .com would have cost $30k and the .org cost $8.95. I can think of no difference it made over the lifetime of the business; my daughter's college fund, on the other hand, has a very clear view of the delta between the two.
[1] n.b. not true of all TLDs; .gov, .edu, and many of the geography-specific ones have various requirements to be able to use them
