The vast majority of Esperantists learn the language as an adult, and unless you are a voracious reader there are few opportunities for practice. Since we normally learn grammar through years of immersion, not instruction, for the most part it is only Esperantists of the academically gifted kind who can look at a page of text and instantly see the grammatical mistakes.
Obviously it is possible for writers to deal with this to a certain extent by asking other people to check their work. However, this is inevitably awkward and creatively constraining. My guess is that songwriters are less likely than most even to feel the need.
It should be remembered of course that the purpose of Esperanto is communication in an atmosphere of freedom and harmony. While obviously we would prefer that grammatical mistakes be few in number (and the grammar is simple for that reason) it is contrary to the spirit of the language to take too much offence at the innocent mistakes of others.