Most proofreading websites simply read your work and correct obvious spelling and grammar errors. They have no idea whether your terminology is correct and they will not even notice incorrect word choices.
This is especially important if you are an LLM or a PhD student who speaks English as a second language (ESOL). You cannot always discern the correct word for a particular legal context since you are not completely familiar with English legal terminology. Even specialist translators sometimes choose the wrong word for a particular legal context because they are translating – they lack the knowledge about the law in English to choose the correct terminology for the correct context.
Rejected by examiners
We often re-edit thesis that has been proofread by other proofreading websites. Most proofreaders fail to comply with the OSCOLA or the Blue Book when checking your referencing—because it is tricky and time-consuming—and thus your thesis will not likely proceed to an oral examination until those errors are corrected.
Being a native English speaker is not a qualification for editing and proofreading a legal thesis.