The problem with the, 'ways around that', may lie in the manner in which extracurricular flying might affect one's employer's insurance liability?
Furthermore, once engaged in such extraneous flying and discovered so to be doing, the pilot might be viewed by his employer as being in breach of contract. Were this to be the case then presumably the employer might be justified in terminating such employment on the grounds that the conditions under which it had been offered had been breached?
It's doubtful that there is actually spite an malice afoot in a company restricting its pilots to fly only for itself.